the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Jul 2022
Seasonal variation of mercury concentration of ancient olive groves of Lebanon
Abstract. This study aimed to investigate the olive, iconic tree of the Mediterranean basin, seasonality of (Hg) mercury pollution. Hg concentrations of foliage, stems, soil surface, and litter were analyzed on monthly basis in ancient olive trees growing in two groves in Lebanon, Bchaaleh and Kawkaba (1300 and 672 m.a.s.l respectively). A significantly lower concentration was registered in stems (~7–9 ng/g) with respect to foliage (~35–48 ng/g) in both sites with the highest foliage Hg concentration in late winter-early spring and the lowest in summer. It is noteworthy that olive fruits also have the lowest Hg concentration (~7–11 ng/g). The soil has the highest Hg content (~62–129 ng/g) likely inherited through the cumulated litter biomass (~ 63–76 ng/g). good covariation observed between our foliage Hg time-series analysis and those of pCO2 and Hg concentrations of the atmospheric Northern hemisphere confirms that mercury pollution can be studied through olive trees. More precisely, spring sampling is recommended if the objective is to assess the tree's susceptibility to Hg uptake. This may draw an adequate baseline for global inventories on Hg vegetation uptake and for new studies on olive trees in the Mediterranean for reconstructing regional Hg pollution concentrations in the past and present.
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RC1: 'Comment on egusphere-2022-174', Håkan Pleijel, 11 Aug 2022
This manuscript contains interesting new information, which should be of interest to the broader scientific audience. The Hg concentration of leaves, fruits, stems, litter of olive trees and soil (at different depths) in two geographically separated olive groves in Lebanon was investigated. Very little information on this is available in the scientific literature. Importantly the seasonal dynamics of the Hg concentrations of the different compartments were investigated. The relative importance of root uptake vs. uptake by leaves from the atmosphere is discussed.
I have two main concerns with the manuscript. Firstly, the language and clarity in several statements should be revised to improve the presentation. Please find below a number of suggestions of possible improvements. A native English speaker should check the entire manuscript. Secondly, the phenology of leaf dynamics in Olea europaea has been neglected in the interpretation of seasonal variation in leaf concentrations of Hg. According to literature a new flush of leaves emerges in spring in olive trees, while leaves are perennial and remain on the tree approximately 3 years. The new leaves that emerge and grow in spring and early summer have not been exposed to Hg to any large extent. Thus, they can be expected to start their development with very low concentrations of Hg – the Hg level increases rather monotonically with leaf age in many tree species (see Wohlegemuth et al 2021, Pleijel et al 2021). Thus, leaves sampled after the new flush of leaves has emerged and grown will typically have a lower Hg concentration than before the new leaves have emerged and become fully developed. Wouldn´t such a “dilution” effect explain (at least part of) the seasonal dynamics in foliage Hg concentration presented in Figure 2 a and b? If so, it would also influence the interpretation of the seasonal dynamics in the Discussion. The influence of the leaf phenology dynamics on foliage Hg concentration needs to be discussed to improve the quality of the manuscript.
Detailed comments:
Lines 21-22: clearer like this: “… to investigate the seasonality of the mercury (Hg) concentration of olive, an iconic tree …”.
Line 22: please change “was” to “were”.
Line 27: maybe “low” rather than “lowest” since stems seem to have had equally low or lower concentration (line 24).
Line 30: “global”? Rather it is relevant for the kind of vegetation typical of the Mediterranean basin and similar climates, which is in itself important.
Line 38: “most important” or “most widely distributed” rather than “most common”?
Line 51: maybe simplify – “Forests are known to act …”.
Line 52: rather “leaf gas exchange” than “photosynthesis”.
Line 54-55: maybe – “… or transferred to other plant organs …”.
Line 60: replace “is said” by “has been estimated”?
Line 64: “earth”? Do you mean “terrestrial”?
Line 68: is it really through “photo-respiration”? I think not. Please be more specific about the relevant “biological processes” referred to.
Line 69: “… Hg exchange between …”.
Line 72-73: The sentence starting “Differences …” is not clear.
Line 94: maybe clearer to say “an air pollution emission area”?
Line 111: “outcomes and consumed”? Not clear.
Lines 114-115: then sentence starting “In addition …” is not complete.
Line 117-119: the objectives would become more informative if they were further elaborated and detailed. The first one could include the information that leaves, fruits, stem, litter and soil were investigated (or this could be an additional, first objective – the comparison of the strongly contrasting Hg concentration levels between the different fractions is a very important aspect of the study). The second one is incomplete and should express that the significance of soil uptake is assessed in relation to uptake by the leaves of Hg from the atmosphere. The sites are locally uncontaminated but considering the scale of the Hg problem a certain degree of contamination occurs over wide areas.
Line 142: it does not become clear if Chekka town is a source of Hg emission or only of other pollutants.
Line 143: “monoxide” should be “carbon monoxide”? “particulate matter”.
Line 224: the sentence is not well phrased.
Line 247-249: why is not the concentrations of fruits included in these comparisons?
Line 266-267: the sentence starting “Seasonal effect …” is not complete.
Line 282 and section 3.3: it does not become completely clear what is compared. Variation among trees in different parts of the investigated areas? The presentation needs improvement in this section.
Line 298: if the p-value is 0.013 it is <0.05 and can thus be considered significant, contrary to what is said in the text!? Why is only one p-value provided if both foliage and stems are tested vs temperature?
Line 343: it could be discussed further why litter typically has higher concentrations than leaves. It should be mentioned here that the leaves shed as litter are likely to mostly be the oldest leaves, which have accumulated Hg during the longest period of time and thus have higher Hg concentrations than the remaining leaves have on average since they consist of both younger and older leaves. It should also be kept in mind that litterfall could have lost organic carbon, thereby concentrating Hg (e.g., Pokharel and Obrist, 2011).
Line 353: rather than “photosynthetic activity and stomatal conductance” it would be more accurate to say “accumulation in leaves after stomatal uptake”?
Line 440: since the distribution of Hg pollution is by nature geographically very widespread, long-distance contamination occurs and it may be better to say “In sites without local contamination” instead of “In uncontaminated sites”. Similarly on line 452 it would be appropriate to say “locally uncontaminated” rather than “non-contaminated”.
Line 459: “but it can also be due to the physiology” – what aspects of physiology? Photosynthetic activity and stomatal conductance already mentioned on line 458 are physiological processes! What other physiological processes do the authors refer to?
Line 467: “… between Hg soil due …”? Difficult to understand.
Why are some references in capitals, e.g., lines 512-513, lines 521-523, lines 712-715. Pleijel et al and Wohlgemuth et al are no longer preprints.
Font size in figures should be increased to improve readability.
In general, the results are interesting and important. They should be published, but as already mentioned the Hg concentration of foliage in relations to phenological dynamics of olive tree leaves needs to be considered in the discussion of the results.
Reference:
Pokharel, A. K. and Obrist, D.: Fate of mercury in tree litter during decomposition, Biogeosciences, 8, 2507–2521, https://doi.org/10.5194/bg-8-2507-2011, 2011.
Citation: https://doi.org/10.5194/egusphere-2022-174-RC1 -
AC1: 'Reply on RC1', Nagham Tabaja, 01 Oct 2022
We are very thankful for your constructive comments and feedback on the manuscript submitted. We accepted all the comments provided and amended them accordingly.
Concerning the language and clarity in several statements were revised and improved. A native English speaker checked the manuscript and helped in improving the form. In regards to the phenology of leaf dynamics, it has been indicated that the collected samples were merged from three different years and merged for analysis. In addition the physiology has been more integrated in the text but of course more detailed work can be made separetly to focus on this aspect.
Line 21-22: It is corrected.
Line 22: It is corrected.
Line 27: It is corrected by “It is noteworthy that olive fruits also have low Hg concentration (~7-11 ng/g).”
Line 30: It is corrected by “This may draw an adequate baseline for Eastern Mediterranean and region with similar climate inventories on Hg vegetation uptake and new studies on olive trees in the Mediterranean to reconstruct regional Hg pollution concentrations in the past and present.”
Line 38: It is corrected by “Mercury (Hg) is among the most widely distributed heavy metals polluting the Earth (Briffa et al. 2020).”
Line 51: It is corrected by “Forests are known to act as a sink of atmospheric Hg in the ecosystem.”
Line 52: It is corrected by “Plant foliage take up of Hg deposited on leaf surfaces through the stomata and leaf cuticles”.
Line 54-55: It is corrected by “where it accumulates with minimal mobility and small portions released back into the atmosphere or transferred to other plant organs”
Line 60: It is replaced from “is said” by “has been estimated”.
Line 64: It is replaced from “earth” by “terrestrial”.
Line 68: It is corrected by “Soil can release Hg to the atmosphere (Luo et al., 2016; Yang et al., 2018a; Assad, 2017; Schneider et al., 2019; Gworek et al., 2020; Pleijel et al., 2021) and also behave as a source of Hg to the plants.”
Line 69: It is corrected by “Trees are hence considered as important drivers of Hg exchange between the atmosphere and the soil (Yang et al. 2018) .”
Line 72-73: The sentence starting “Differences …” is not clear. This sentence is removed.
Line 94: It is corrected by “an air pollution emission area.”
Line 111: “outcomes and consumed”? Not clear. It is replaced by “for consumption.”
Lines 114-115: then sentence starting “In addition …” is not complete. This sentence is removed.
Line 117-119: The objectives are amended as per the feedback received. “In this study two sites, known for their century-old olive groves and located at two different altitudes in Lebanon, were selected to assess the Hg contents. In these remote areas, no direct sources of mercury contamination are reported and hence we expect very low Hg concentrations. However, due to atmospheric transport of Hg, deposition can be expected in remote areas (Grigal, 2003). The main objectives of this study are to examine and compare Hg levels in foliage, stems, fruits, litter and soil measured in each of these two olive groves, which we monitored monthly for 18 months. The second objective is to analyze the relative importance of Hg uptake from the soil to the foliage in comparison with the assimilation of atmospheric Hg by the leaves. Is the uptake of Hg from soil to foliage low in sites without local Hg contamination? Since the distribution of Hg pollution is by nature geographically widespread, and given the extent of Hg pollution in the Mediterranean and the transfer of pollution by wind and the Mediterranean Sea, long-distance contamination occurs over large areas. This study will fill the data gap on Hg pollution in the Eastern Mediterranean and may draw an adequate baseline for Eastern Mediterranean and region of similar climates inventories on Hg vegetation uptake and new studies on olive trees in the Mediterranean to reconstruct regional Hg pollution concentrations in the past and present.”
Line 142: it does not become clear if Chekka town is a source of Hg emission or only of other pollutants. This sentence is cleared by “To our knowledge no direct Hg pollution is reported at Chekka and Selaata sites.”.
Line 143: It is corrected by “carbon monoxide”.
Line 224: It is corrected by “For the statistical analysis we used the R 4.1.0 program. Our data are not normally distributed, so for the effect of tissue type on Hg concentration, Wilcoxon test was used with the tissue type (foliage and stems) as the main effect.”
Line 247-249: why is not the concentrations of fruits included in these comparisons? The fruits are added.
Line 266-267: It is corrected by “A seasonal effect on foliage and stems was registered”.
Line 282 and section 3.3: It is rearranged and corrected by “3.3. Tree comparison
In the upper terrace of BC grove, the foliage average Hg concentration of BCO4 and BCO1 varied between 42.4 ± 11.5 ng/g and 44.6 ± 13.3 ng/g respectively showing no significant difference (p-value = 0.8225). In the lower terrace of the same site, foliage average Hg concentrations of trees BCO12 and BCO9 were found to vary from 45.6 ± 12.7 ng/g to 60.7 ± 12.7 ng/g respectively) (Figure 2a,c). Significant differences are shown between foliage of the tree BCO9 and BCO1 (p-value= 0.0019), BCO4 and BCO12, p-value=0.00047) respectively).
In the upper terrace of BC grove, the stems average Hg concentration of BCO4 and BCO1 varied between 7.0 ± 2.8 ng/g and 7.1 ± 2.9 ng/g respectively showing no significant difference (p-value= 0.94). In the lower terrace, stems average Hg concentrations of BCO12 and BCO9 are 6.4 ± 2.2 ng/g nd 11.2 ± 5.2 ng/g respectively showing a significant difference of p-value= 0.0054 (Figure 2a,c). For BCO1 and BCO12 there was no significance difference registered with p-Value= 0.5725, the same goea for BCO4 and BCO12 no significance difference was registered with p-value= 0.523.
The average concentration per tree in foliage and stems were 32.4 ± 12.2 ng/g and 8.5 ± 4.0 ng/g respectively for KWO1, 32. 8 ± 14.7 ng/g and 8.9 ± 6.0 ng/g for KWO2, 37.6 ± 14.0 ng/g and 9.3 ± 6.7 ng/g for KWO3 and 37.7 ± 13.6 ng/g and 9.6 ± 4.0 ng/g for KWO4 (Figure 2b,d). In KW grove, comparison of the foliage Hg concentration between the four studied trees shows no significant difference (0.22<p-value<1) so did the stems (0.21<p-value<0.96).
In BC grove, the trees located on the lower terrace recorded higher Hg concentration values than those of the upper terrace especially tree BCO9. While KW grove had similar Hg concentration among all four trees.”
Line 298: It is corrected and values of foliage and stems are separated.
Line 343: It is amended by “In parallel, the litter showed higher Hg concentration than that in foliage in both BC (62.9 ± 17.8 ng/g) and KW (75.7 ± 20.3 ng/g) (Table 1) likely explained by the process of the Hg input into the litter through foliage shedding its oldest foliage to the forest floor which have accumulated Hg during the longest period of time and thus have higher Hg concentrations than the remaining foliage have on average since they consist of both younger and older foliage (Rea et al. 1996; Pleijel et al. 2021).”
Line 353: It is corrected by “accumulation in leaves after stomatal uptake.”
Line 440: since the distribution of Hg pollution is by nature geographically very widespread, long-distance contamination occurs and it may be better to say “In sites without local contamination” instead of “In uncontaminated sites”. Similarly on line 452 it would be appropriate to say “locally uncontaminated” rather than “non-contaminated”. It is corrected,
Why are some references in capitals, e.g., lines 512-513, lines 521-523, lines 712-715. Pleijel et al and Wohlgemuth et al are no longer preprints. Those references are corrected.
Font size in figures should be increased to improve readability. It is amended.
Citation: https://doi.org/10.5194/egusphere-2022-174-AC1
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AC1: 'Reply on RC1', Nagham Tabaja, 01 Oct 2022
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RC2: 'Comment on egusphere-2022-174', Anonymous Referee #4, 25 Aug 2022
This data set explores Hg uptake and cycling in olive groves – an critical agricultural tree across much of the Mediterranean basin. The experimental design seems valid to conduct an initial examination of Hg physiological cycling within olive trees and their surrounding soils. Additionally, the QA/QC of the data seems valid, which provides merit to at least the data.
Nonetheless, I cannot support publication of this manuscript in its current form for the reasons listed below. My recommendations would be to considerably scale back the scope of this manuscript and limit the discussion to the bounds of the data (extravagant speculations based on distant and unrelated data need to be removed), work on the grammar and English language of the paper, and consider the structural suggestions made below. At this point the paper may be worthy of resubmission and publication. Whether that publication is worth of publication in EGU-Sphere is a decision for the editor(s).
General comments:
- I feel like this paper would greatly benefit from combining the results and discussion. The current separated results and discussion lead to very dry reading that makes engagement somewhat difficult and there is clear repetition in the discussion section on what has already been written in the results. List then data and discuss it along in context of the story the manuscript is attempting to make. Additionally, there is repetition in the listing of the data between the text, figure 2 and table 2 (and the discussion). These data need only be listed one time.
- There is a lot of speculation and much of this speculation appears unsupported or poorly supported by the available data; the discussion oversteps the suggestions and conclusions that can be made with this dataset via the excessive speculation. (a) There is a whole section looking at correlations between atmospheric Hg(0) from Europe (specifically it would seems Schauinsland, Germany; And data from 7 years prior) and foliage concentrations in these Lebanese olive groves, which are completely different ecological biomes and climatic zones. The spatiotemporal differences between these datasets is vast. (b) The authors jump between soils being a source of Hg to aerial parts of the plant and back to the atmosphere being the source without any definitive evidence that can clearly define which source is the actual source. Suggestions of soils sources would also contradict basically all (and a very large pool of) evidence that suggests this uptake pathway is minimal in trees. If the authors want to suggest this is a significant pathway they need strong evidence to do so and this study does not provide such evidence.
- I also believe the suggestion by another review that new foliage emergence is the main driver of the observed seasonal differences in foliar mercury is valid and needs detailed consideration.
Line 38-39: The term heavy metals is a poor descriptor and one that has been suggested multiple times to be made redundant (Duffus, 2009: https://doi.org/10.1515/ci.2001.23.6.163; Pourrett and Hursthouse, 2019: 10.3390/ijerph16224446 ). I would suggest changing the terminology throughout with a less ambiguous descriptor like “potentially toxic metals”.
Lines 44-45: change to “Hg(0) is primarily transferred through the atmosphere by air mass movement and can undergo long-range transport”.
Lines 45-47: This is incorrect as written. Hg(0) does not “covalently bond with organic groups to forming… MeHg”. It must first be oxidized (either in the atmosphere or in terrestrial matrices after deposition), transferred to anoxic or poorly oxic conditions and it can then be methylated.
Lines 47-48: These descriptions about legacy mercury are extremely vague and need to be improved. It is also a bit out of place with the rest of the story and I think these two sentences could be deleted without effect.
Lines 48-50: Needs grammatical correction.
Lines 51: Delete “in the ecosystem”.
Lines 51-63: This paragraph needs grammatical and structural (and English language) work. It is a bit disjointed and jumps from one thought to another continuously.
Lines 67-69: This ignores one of the most critical fluxes of Hg back to the atmosphere from forests: wildfires. Please add a statement on this and include references such as: McLagan et al. (2021) 10.5194/acp-638 21-5635-2021; Dastoor et al. (2022) 10.1038/s43017-022-00269-w; Friedli et al. (2009) 10.1021/es802703g.
Lines 73-76: I disagree with this statement. Tree ring Hg (dendrochronology) is predominantly used as an archiving tool for atmospheric Hg(0) (Hg(0) oxidised in leaves, transferred in phloem to bole wood, and generally considered to be stored long-term). It has been established for decades (Beauford et al., 1977: 0.1111/j.1399-3054.1977.tb01880.x; Lindberg et al. (1979) 10.2134/jeq1979.00472425000800040026x) and re-confirmed many times since that Hg in woody materials is derived from atmosphere. Please correct these statements accordingly.
Lines 76-77: I would suggest to add McLagan et al., (2022: https://doi.org/10.5194/bg-2022-124, recently accepted) to this reference on Hg dendrochemistry (using stable Hg isotopes). Some of the findings in this recently accepted study may be highly beneficial to this manuscript.
Lines 102-103: I cannot agree with this statement that roots are the primary source of Hg in contaminated areas. (1) This is unpublished work and judging from the abstract it appears they state the atmosphere as the source not the roots; (2) This is at a former Hg mine – there is MASSIVE legacy emissions of Hg(0) to the atmosphere continuing to this day at these sites, which is readily available for stomatal assimilation; (3) as previously mentioned there is countless studies during the past 50 years that show root uptake in tree ubiquitously is an very minor, if not insignificant uptake pathway. This statements needs correcting.
Lines 107-108: How does this compare to recommended soil guidelines? Please state this.
Lines 144: I really don’t see the benefit of making acronyms of the sampling sites. Both a one-word towns and this just confuses readers that are not as closely linked to the study as the authors. I recommend simply writing the town names each time.
Figure 1: The climate graphs are really ancillary metadata. These are described in the method text and should be moved to the SI. Indeed, even the site map could be move to the supplementary information (SI). There are only two studies sites and again their location, climate and geography and surrounding Hg sources are described in the text. I believe this whole figure would be better served in a SI.
Line 177: “8-15 m foot circumference”. There are obvious errors here. Also I highly recommend using diameter rather than circumference. It is much easier for the reader to comprehend.
Line 196-197: While I do not think this is a major problem as I believe there will be minimal Hg(0) on surfaces or within foliage and stems, it needs to be acknowledge that this heated drying method would likely eliminated any and ALL Hg(0) present in the samples.
Line 217: The detection limit should be listed as total mass of Hg, not concentration of Hg. The system does not analyze concentration (that is calculated by the mass of sample input), it is calibrated to determine the mass of Hg in any given sample. This is an important distinction.
Line 261-263: What Hg concentration is being referred to here? Hg(0) Concentration in the air? This needs to be stated. This ambiguity is exactly why the results and discussion should be combined.
Lines 312: “the main Hg content” should be changed to “Highest Hg concentration”.
Lines 318-322: Once again, I disagree the main source of Hg in the stems of the trees is from the soils. What is the evidence for this? I could reference 10+ papers that have shown Hg in woody materials of trees to be almost exclusively derived from foliage and downward transport in phloem. Higher concentrations in leaves over stems is NOT evidence that Hg in stems is derived from the roots. Not all Hg taken up by foliage is transferred to woody materials, which leads to an enrichment of Hg in foliage compared to Hg in woody materials.
Lines 323-324: The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils.
Lines 333-335: This concept of soil properties driving Hg concentrations and uptake in soils was not something introduced by O’Connor et al., 2019. This is not a new idea and again has been known of for decades.
Line 336: What does nitrogen content have to do with Hg sorption and uptake in soils?
Lines 339-341: Did the authors measure wet and dry deposition of Hg? For wet deposition to occur, there would need to be considerable Hg(II) in the atmosphere. I also see no reason as to why dry deposition would occur more in a higher temperature region. Higher temperatures favour partitioning of Hg(0) back into the gas phase, which would be suggestive of less dry deposition.
Lines 342-344: Foliage accumulates Hg(0) over time. Naturally older leaves that eventually die and senesce will be more enriched in Hg than younger leaves growing on the trees.
Lines 348-404 and Figure 3: This is far too speculative. These data are for Europe (the Authors use a site from Germany for Hg(0) in Figure 3). Lebanon is a long way from Europe and in a totally different climatic zone without typical northern temperate/boreal deciduous/conifer dominated forests. To make any sort of statement about atmospheric Hg zero concentrations this should have been measured or data taken from a long-term monitoring station in this climatic region/ecological biome. I see this whole (and very long discussion) on correlations between foliage and atmospheric Hg(0) to be too speculative to the point it is invalid. I also agree with the other reviewer that the emergence of foliage in olive trees in spring/early summer is the major driver here.
Lines 417-448: These paragraphs need grammatical and English language corrections. It is very hard to follow and from what I can derive it again seems highly speculative and to contradict the state of the science without data to support that.
Citation: https://doi.org/10.5194/egusphere-2022-174-RC2 -
AC2: 'Reply on RC2', Nagham Tabaja, 02 Oct 2022
Your input to this study is much appreciated and of great help to this work in order to improve it and make it better for publication. Your recommendations were taken into account and the English and grammar were worked on by the help of a scientific English editor.
In this paper we did not combine the results and discussion, following the journal way of dividing the different parts of the article. The repetition that can be found in the discussion is minimal and is to make it easier for the reader to relate more directly without having to go back to the results part that is much smaller in comparison to the discussion.
The repetition between the text and figure and the table is to show the needed information in a more visual way and in a form of values for the reader to be able to be in contact with the reality of the study.
The speculations made are based on the few data found and on our own analysis and point of view on the scientific data. The comparison made with the Northern Hemisphere from data of other countries far from our region is due to the lack of data in the Easter Mediterranean. The main suggestion of the source of the Hg concentration is the atmosphere and a minimal source from the soil that does need more study and dedication.
In this study three different generations of foliage were mixed and studied that may have affected the seasonality in foliar mercury.
Line 38-39: The term heavy metals is a poor descriptor and one that has been suggested multiple times to be made redundant (Duffus, 2009: https://doi.org/10.1515/ci.2001.23.6.163; Pourrett and Hursthouse, 2019: 10.3390/ijerph16224446 ). I would suggest changing the terminology throughout with a less ambiguous descriptor like “potentially toxic metals”. This phrase is corrected.
Lines 44-45: change to “Hg(0) is primarily transferred through the atmosphere by air mass movement and can undergo long-range transport”. This phrase is corrected as suggested.
Lines 45-47: This is incorrect as written. Hg(0) does not “covalently bond with organic groups to forming… MeHg”. It must first be oxidized (either in the atmosphere or in terrestrial matrices after deposition), transferred to anoxic or poorly oxic conditions and it can then be methylated. It is corrected by “This highly diffusive Hgcan easily pass biological barriers (i.e. cell membranes, foliage, skin) and it must first be oxidized (either in the atmosphere or in terrestrial matrices after deposition), transferred to anoxic or poorly oxic conditions and bind covalently with organic groups forming the widespread toxic methylmercury (MeHg, CH3Hg+) (Clarkson and Magos 2006).”
Lines 47-48: These descriptions about legacy mercury are extremely vague and need to be improved. It is also a bit out of place with the rest of the story and I think these two sentences could be deleted without effect. These sentences were deleted.
Lines 48-50: Needs grammatical correction. It was revised by an English scientific editor.
Lines 51: Delete “in the ecosystem”. This word is deleted.
Lines 51-63: This paragraph needs grammatical and structural (and English language) work. It is a bit disjointed and jumps from one thought to another continuously. This paragraph was checked and revised by an English scientific editor.
Lines 67-69: This ignores one of the most critical fluxes of Hg back to the atmosphere from forests: wildfires. Please add a statement on this and include references such as: McLagan et al. (2021) 10.5194/acp-638 21-5635-2021; Dastoor et al. (2022) 10.1038/s43017-022-00269-w; Friedli et al. (2009) 10.1021/es802703g. This phrase is added “An important source of Hg emission to the atmosphere is from the biomass burning, ocean currents and rivers and is also considered an essential component of the Hg global biochemical cycle (Friedli et al., 2009; McLagan et al., 2021; Dastoor et al., 2022).”
Lines 73-76: I disagree with this statement. Tree ring Hg (dendrochronology) is predominantly used as an archiving tool for atmospheric Hg(0) (Hg(0) oxidised in leaves, transferred in phloem to bole wood, and generally considered to be stored long-term). It has been established for decades (Beauford et al., 1977: 0.1111/j.1399-3054.1977.tb01880.x; Lindberg et al. (1979) 10.2134/jeq1979.00472425000800040026x) and re-confirmed many times since that Hg in woody materials is derived from atmosphere. Please correct these statements accordingly. I used this statement from a new reference from Yanai et al. 2020.
Lines 76-77: I would suggest to add McLagan et al., (2022: https://doi.org/10.5194/bg-2022-124, recently accepted) to this reference on Hg dendrochemistry (using stable Hg isotopes). Some of the findings in this recently accepted study may be highly beneficial to this manuscript. The following phrase is added “Another study that used Hg stable isotopes propose a reemission and reduction of the foliage Hg can also take place of this internal leaf Hg (between 29 and 42 % of gross uptake based on the plant species studied) may occur (Yuan et al., 2018)”.
Lines 102-103: I cannot agree with this statement that roots are the primary source of Hg in contaminated areas. (1) This is unpublished work and judging from the abstract it appears they state the atmosphere as the source not the roots; (2) This is at a former Hg mine – there is MASSIVE legacy emissions of Hg(0) to the atmosphere continuing to this day at these sites, which is readily available for stomatal assimilation; (3) as previously mentioned there is countless studies during the past 50 years that show root uptake in tree ubiquitously is an very minor, if not insignificant uptake pathway. This statements needs correcting. This statement is removed.
Lines 107-108: How does this compare to recommended soil guidelines? Please state this. This phrase is corrected as follows “Adding to that, soil samples collected from different areas in southern Lebanon showed values of Hg concentration ranging between 160-6480 ng/g showing a high contamination levels”.
Lines 144: I really don’t see the benefit of making acronyms of the sampling sites. Both a one-word towns and this just confuses readers that are not as closely linked to the study as the authors. I recommend simply writing the town names each time. This acronomy was made to make it easier on readers to distinguish between sites without having to spell them each time over the study knowing how hard it is to pronounce them.
Figure 1: The climate graphs are really ancillary metadata. These are described in the method text and should be moved to the SI. Indeed, even the site map could be move to the supplementary information (SI). There are only two studies sites and again their location, climate and geography and surrounding Hg sources are described in the text. I believe this whole figure would be better served in a SI. In my opinion i believe it is better for the reader to have those sites indicated directly after the text to make it clearer concerning the locations.
Line 177: “8-15 m foot circumference”. There are obvious errors here. Also I highly recommend using diameter rather than circumference. It is much easier for the reader to comprehend. These data were rechecked and there is no error in the values.
Line 196-197: While I do not think this is a major problem as I believe there will be minimal Hg(0) on surfaces or within foliage and stems, it needs to be acknowledge that this heated drying method would likely eliminated any and ALL Hg(0) present in the samples. This statement was amended to the following “Collected foliage and stems were rinsed with distilled water and then dried for 48 hours in an oven at a temperature of 60°C at maximum in order to remove any dust Hg from the surface of the samples.”
Line 217: The detection limit should be listed as total mass of Hg, not concentration of Hg. The system does not analyse concentration (that is calculated by the mass of sample input), it is calibrated to determine the mass of Hg in any given sample. This is an important distinction. The detection limit is changed to total mass as suggested.
Line 261-263: What Hg concentration is being referred to here? Hg(0) Concentration in the air? This needs to be stated. This ambiguity is exactly why the results and discussion should be combined. The Hg(0) is the referred to Hg concentration.
Lines 312: “the main Hg content” should be changed to “Highest Hg concentration”. This phrase is corrected.
Lines 318-322: Once again, I disagree the main source of Hg in the stems of the trees is from the soils. What is the evidence for this? I could reference 10+ papers that have shown Hg in woody materials of trees to be almost exclusively derived from foliage and downward transport in phloem. Higher concentrations in leaves over stems is NOT evidence that Hg in stems is derived from the roots. Not all Hg taken up by foliage is transferred to woody materials, which leads to an enrichment of Hg in foliage compared to Hg in woody materials. I agree with your argument, but as said minima Hg(0) is transferred from foliage to stems and not all comes from the roots .
Lines 323-324: The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils. This phrase is added : The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils.”
Lines 333-335: This concept of soil properties driving Hg concentrations and uptake in soils was not something introduced by O’Connor et al., 2019. This is not a new idea and again has been known of for decades. I agree, but I am giving here an updated reference.
Line 336: What does nitrogen content have to do with Hg sorption and uptake in soils? Nitrogen can also be a factor affecting the Hg content in soil depending on its characteristics.
Lines 339-341: Did the authors measure wet and dry deposition of Hg? For wet deposition to occur, there would need to be considerable Hg(II) in the atmosphere. I also see no reason as to why dry deposition would occur more in a higher temperature region. Higher temperatures favour partitioning of Hg(0) back into the gas phase, which would be suggestive of less dry deposition. The author discussed dry deposition and indicated the effect of higher temperature on higher dry deposition.
Lines 342-344: Foliage accumulates Hg(0) over time. Naturally older leaves that eventually die and senesce will be more enriched in Hg than younger leaves growing on the trees. I agree with your statement.
Lines 348-404 and Figure 3: This is far too speculative. These data are for Europe (the Authors use a site from Germany for Hg(0) in Figure 3). Lebanon is a long way from Europe and in a totally different climatic zone without typical northern temperate/boreal deciduous/conifer dominated forests. To make any sort of statement about atmospheric Hg zero concentrations this should have been measured or data taken from a long-term monitoring station in this climatic region/ecological biome. I see this whole (and very long discussion) on correlations between foliage and atmospheric Hg(0) to be too speculative to the point it is invalid. I also agree with the other reviewer that the emergence of foliage in olive trees in spring/early summer is the major driver here. Another data done on the western Mediterranean showed the same results as we implicated in our study (Mastromonaco et al. 2017). Concerning this part, the problem is that there is no data related to the Eastern Mediterranean that we could find to use as a comparison in our study. But this can be a starting point to have more data collection that is closer to our studied regions.
Lines 417-448: These paragraphs need grammatical and English language corrections. It is very hard to follow and from what I can derive it again seems highly speculative and to contradict the state of the science without data to support that. These paragraphs were checked by a scientific English editor. Some data was given to support our speculations, but we tried to give our point of view in regards to the Hg cycling n stems, litter and soil system.
Citation: https://doi.org/10.5194/egusphere-2022-174-AC2 -
AC3: 'Reply on RC2', Nagham Tabaja, 11 Oct 2022
Please disregard the first response and check the second response to the comments given.
Response to the comment on EGUsphere-2022-174 by Hakan Pleijel (Referee 1) on the article “ Seasonal variation of mercury concentration of ancient olive groves of Lebanon” by Tabaja et al. https://doi.org/10.5194/egusphere-2022-174-RC1, 2022
We are very thankful for your constructive comments and feedback on the manuscript submitted. We accepted all the comments provided and amended them accordingly.
Concerning the language and clarity in several statements were revised and improved. A native English speaker checked the manuscript and helped in improving the form. In regards to the phenology of leaf dynamics, it has been indicated that the collected samples were merged from three different years and merged for analysis. The phenological growth stages of olive trees described by Sanz-Cortès et al. (2002) in Spain suggest leaf development from March to November. Hence the Hg concentration measured on monthly collected leaves represents an average of Hg accumulated in young leaves (year N of collection) and older leaves (N-1 year and N-2 years) where N is equal to 2019 and 2020. According to Pleijel et al. 2021, young leave has less mercury than older leaves generations. We can expect that our Hg concentration is lower during the growth of young leaves due to the Hg dilution signal with the low Hg content of the younger leaves. In a very simple approach, we can suggest the late winter-early spring should exhibit the lowest Hg contents. However, our results show the lowest values in summer when young leaves have already grown significantly. In addition, physiology has been more integrated into the text but of course, more detailed work can be made separately to focus on this aspect.
Line 21-22: It is corrected.
Line 22: It is corrected.
Line 27: It is corrected by “It is noteworthy that olive fruits also have low Hg concentration (~7-11 ng/g).”
Line 30: It is corrected by “This may draw an adequate baseline for Eastern Mediterranean and region with similar climate inventories on Hg vegetation uptake and new studies on olive trees in the Mediterranean to reconstruct regional Hg pollution concentrations in the past and present.”
Line 38: It is corrected by “Mercury (Hg) is among the most widely distributed heavy metals polluting the Earth (Briffa et al. 2020).”
Line 51: It is corrected by “Forests are known to act as a sink of atmospheric Hg in the ecosystem.”
Line 52: It is corrected by “Plant foliage take up of Hg deposited on leaf surfaces through the stomata and leaf cuticles”.
Line 54-55: It is corrected by “where it accumulates with minimal mobility and small portions released back into the atmosphere or transferred to other plant organs”
Line 60: It is replaced from “is said” by “has been estimated”.
Line 64: It is replaced from “earth” by “terrestrial”.
Line 68: It is corrected by “Soil can release Hg to the atmosphere (Luo et al., 2016; Yang et al., 2018a; Assad, 2017; Schneider et al., 2019; Gworek et al., 2020; Pleijel et al., 2021) and also behave as a source of Hg to the plants.”
Line 69: It is corrected by “Trees are hence considered as important drivers of Hg exchange between the atmosphere and the soil (Yang et al. 2018).”
Line 72-73: The sentence starting “Differences …” is not clear. This sentence is removed.
Line 94: It is corrected by “an air pollution emission area.”
Line 111: “outcomes and consumed”? Not clear. It is replaced by “for consumption.”
Lines 114-115: then sentence starting “In addition …” is not complete. This sentence is removed.
Line 117-119: The objectives are amended as per the feedback received. “In this study two sites, known for their century-old olive groves and located at two different altitudes in Lebanon, were selected to assess the Hg contents. In these remote areas, no direct sources of mercury contamination are reported and hence we expect very low Hg concentrations. However, due to atmospheric transport of Hg, deposition can be expected in remote areas (Grigal, 2003). The main objectives of this study are to examine and compare Hg levels in foliage, stems, fruits, litter and soil measured in each of these two olive groves, which we monitored monthly for 18 months. The second objective is to analyze the relative importance of Hg uptake from the soil to the foliage in comparison with the assimilation of atmospheric Hg by the leaves. Is the uptake of Hg from soil to foliage low in sites without local Hg contamination? Since the distribution of Hg pollution is by nature geographically widespread, and given the extent of Hg pollution in the Mediterranean and the transfer of pollution by wind and the Mediterranean Sea, long-distance contamination occurs over large areas. This study will fill the data gap on Hg pollution in the Eastern Mediterranean and may draw an adequate baseline for Eastern Mediterranean and region of similar climates inventories on Hg vegetation uptake and new studies on olive trees in the Mediterranean to reconstruct regional Hg pollution concentrations in the past and present.”
Line 142: it does not become clear if Chekka town is a source of Hg emission or only of other pollutants. This sentence is cleared by “To our knowledge no direct Hg pollution is reported at Chekka and Selaata sites.”.
Line 143: It is corrected by “carbon monoxide”.
Line 224: It is corrected by “For the statistical analysis we used the R 4.1.0 program. Our data are not normally distributed, so for the effect of tissue type on Hg concentration, Wilcoxon test was used with the tissue type (foliage and stems) as the main effect.”
Line 247-249: why is not the concentrations of fruits included in these comparisons? The fruits are added.
Line 266-267: It is corrected by “A seasonal effect on foliage and stems was registered”.
Line 282 and section 3.3: It is rearranged and corrected by “3.3. Tree comparison
In the upper terrace of BC grove, the foliage average Hg concentration of BCO4 and BCO1 varied between 42.4 ± 11.5 ng/g and 44.6 ± 13.3 ng/g respectively showing no significant difference (p-value = 0.8225). In the lower terrace of the same site, foliage average Hg concentrations of trees BCO12 and BCO9 were found to vary from 45.6 ± 12.7 ng/g to 60.7 ± 12.7 ng/g respectively) (Figure 2a,c). Significant differences are shown between foliage of the tree BCO9 and BCO1 (p-value= 0.0019), BCO4 and BCO12, p-value=0.00047) respectively).
In the upper terrace of BC grove, the stems average Hg concentration of BCO4 and BCO1 varied between 7.0 ± 2.8 ng/g and 7.1 ± 2.9 ng/g respectively showing no significant difference (p-value= 0.94). In the lower terrace, stems average Hg concentrations of BCO12 and BCO9 are 6.4 ± 2.2 ng/g nd 11.2 ± 5.2 ng/g respectively showing a significant difference of p-value= 0.0054 (Figure 2a,c). For BCO1 and BCO12 there was no significance difference registered with p-Value= 0.5725, the same goea for BCO4 and BCO12 no significance difference was registered with p-value= 0.523.
The average concentration per tree in foliage and stems were 32.4 ± 12.2 ng/g and 8.5 ± 4.0 ng/g respectively for KWO1, 32. 8 ± 14.7 ng/g and 8.9 ± 6.0 ng/g for KWO2, 37.6 ± 14.0 ng/g and 9.3 ± 6.7 ng/g for KWO3 and 37.7 ± 13.6 ng/g and 9.6 ± 4.0 ng/g for KWO4 (Figure 2b,d). In KW grove, comparison of the foliage Hg concentration between the four studied trees shows no significant difference (0.22<p-value<1) so did the stems (0.21<p-value<0.96).
In BC grove, the trees located on the lower terrace recorded higher Hg concentration values than those of the upper terrace especially tree BCO9. While KW grove had similar Hg concentration among all four trees.”
Line 298: It is corrected and values of foliage and stems are separated.
Line 343: It is amended by “In parallel, the litter showed higher Hg concentration than that in foliage in both BC (62.9 ± 17.8 ng/g) and KW (75.7 ± 20.3 ng/g) (Table 1) likely explained by the process of the Hg input into the litter through foliage shedding its oldest foliage to the forest floor which have accumulated Hg during the longest period of time and thus have higher Hg concentrations than the remaining foliage have on average since they consist of both younger and older foliage (Rea et al. 1996; Pleijel et al. 2021).”
Line 353: It is corrected by “accumulation in leaves after stomatal uptake.”
Line 440: since the distribution of Hg pollution is by nature geographically very widespread, long-distance contamination occurs and it may be better to say “In sites without local contamination” instead of “In uncontaminated sites”. Similarly on line 452 it would be appropriate to say “locally uncontaminated” rather than “non-contaminated”. It is corrected.
Why are some references in capitals, e.g., lines 512-513, lines 521-523, lines 712-715. Pleijel et al and Wohlgemuth et al are no longer preprints. Those references are corrected.
Font size in figures should be increased to improve readability. It is amended.
Citation: https://doi.org/10.5194/egusphere-2022-174-AC3 -
AC4: 'Reply on AC3', Nagham Tabaja, 12 Oct 2022
Please disregard the previous response and check instead the below.
Response to the comment on EGUsphere-2022-174 by Anonymous Referee on the article “ Seasonal variation of mercury concentration of ancient olive groves of Lebanon” by Tabaja et al. https://doi.org/10.5194/egusphere-2022-174-RC1, 2022
Thank you very much for the pertinent revision of our manuscript. Your input to this study is much appreciated and of great help to this work in order to improve it and make it better for publication. Your recommendations were taken into account and the English and grammar were revised by the help of a scientific English editor.
In this paper we did not combine the results and discussion, following the journal mostly used way of dividing the different parts of the article. The repetition that can be found in the discussion is minimal and is to make it easier for the reader to relate more directly without having to go back to the results part which is much smaller in comparison to the discussion.
The repetition between the text and figure and the table have been minimized.
The speculations made have been revised. We removed the comparison made with data of other countries far from our region. In our study, the main suggestion of the source of the Hg concentration is the atmosphere while only a minimal source from the soil was considered as reflected in the manuscript.
In this study three different generations of foliage were mixed and studied. We are aware that this certainly affected the seasonality in foliar mercury. This has been discussed following the rev 1 comments. We included in the text “For each olive tree, both sun exposed and shaded foliage (olive tree bears foliage from three different years -age classes current+2 (C+2) was collected during the same year) and stems (terminal portions of 20 cm) with no evidence of pathogens were randomly taken and merged from the upper, middle, and lower canopy position of the olive trees on a monthly basis using a manual pruner.”
Line 38-39: The term heavy metals is a poor descriptor and one that has been suggested multiple times to be made redundant (Duffus, 2009: https://doi.org/10.1515/ci.2001.23.6.163; Pourrett and Hursthouse, 2019: 10.3390/ijerph16224446 ). I would suggest changing the terminology throughout with a less ambiguous descriptor like “potentially toxic metals”. This phrase is corrected as follows: “Mercury (Hg) is among the most widely distributed potentially toxic metals polluting the Earth (Briffa et al. 2020)”.
Lines 44-45: change to “Hg(0) is primarily transferred through the atmosphere by air mass movement and can undergo long-range transport”. This phrase is corrected as suggested: “Hg(0) is primarily transferred through the atmosphere by air mass movement and can undergo long-range transport”.
Lines 45-47: This is incorrect as written. Hg(0) does not “covalently bond with organic groups to forming… MeHg”. It must first be oxidized (either in the atmosphere or in terrestrial matrices after deposition), transferred to anoxic or poorly oxic conditions and it can then be methylated. It is corrected by “Because of its high volatility and susceptibility to oxidation, elemental Hg(0) is the predominant form of Hg in the atmosphere. This highly diffusive Hg can easily pass biological barriers (i.e. cell membranes, foliage, skin). Mercury has three oxidation states, namely, Hg(0) (elemental mercury), Hg(I) (mercurous), or Hg(II) (mercuric), although Hg(I) mercurous form is not stable under typical environmental conditions and, therefore, is rarely observed. It is likely that the Hg(II) high binding affinities bind covalently with organic groups to forming the widespread toxic methylmercury (MeHg, CH3Hg+) (Du and Fang, 1983; Clarkson and Magos 2006; Pleijel et al., 2021).”
Lines 47-48: These descriptions about legacy mercury are extremely vague and need to be improved. It is also a bit out of place with the rest of the story and I think these two sentences could be deleted without effect. These sentences were deleted.
Lines 48-50: Needs grammatical correction. It was revised by an English scientific editor.
Lines 51:Delete “in the ecosystem”. This word is deleted.
Lines 51-63: This paragraph needs grammatical and structural (and English language) work. It is a bit disjointed and jumps from one thought to another continuously. This paragraph was checked and revised by an English scientific editor.
Lines 67-69: This ignores one of the most critical fluxes of Hg back to the atmosphere from forests: wildfires. Please add a statement on this and include references such as: McLagan et al. (2021) 10.5194/acp-638 21-5635-2021; Dastoor et al. (2022) 10.1038/s43017-022-00269-w; Friedli et al. (2009) 10.1021/es802703g. This phrase is added “Though variable from year to year, Hg emission to the atmosphere from biomass burning is considered as an important driver of the global Hg biogeochemical cycle ( Friedli et al., 2009; De Simone et al., 2015; McLagan et al., 2021; Dastoor et al., 2022)”.
Lines 73-76: I disagree with this statement. Tree ring Hg (dendrochronology) is predominantly used as an archiving tool for atmospheric Hg(0) (Hg(0) oxidised in leaves, transferred in phloem to bole wood, and generally considered to be stored long-term). It has been established for decades (Beauford et al., 1977: 0.1111/j.1399-3054.1977.tb01880.x; Lindberg et al. (1979) 10.2134/jeq1979.00472425000800040026x) and re-confirmed many times since that Hg in woody materials is derived from atmosphere. Please correct these statements accordingly. The studies of the Hg cycle in forest ecosystems show that gaseous elemental Hg(0) is the main source taken up by plants (Bishop et al. 2020; Zhou et al. 2021). Analysis of long term atmospheric Hg(0) and CO2 concentrations are very informative to understand the role of the vegetation in the global Hg cycle (Jiskra et al. 2018). Emission reduction measures adopted in Europe and North America since the 70s are corroborated by Hg dendrochemistry analysis showing a declining Hg concentration trend from the older to newer tree rings (Yanai et al., 2020). Indeed, tree ring Hg (dendrochronology) is a powerful archiving tool for atmospheric Hg(0). After Hg(0) oxidation inside the leaves, Hg(II) bind to organic compounds and then is transported fo the bole wood via the phloem (Beaufort et al., 1977; Lindberg et al., 1979). This is corroborated by the recent study of McLagan et al. (2022) showing the benefit of the stable Hg isotope analysis on dendrochrmistry.
Lines 76-77: I would suggest to add McLagan et al., (2022: https://doi.org/10.5194/bg-2022-124, recently accepted) to this reference on Hg dendrochemistry (using stable Hg isotopes). Some of the findings in this recently accepted study may be highly beneficial to this manuscript. We thank you for the reference. We added it as shown in the previous section.
Lines 102-103: I cannot agree with this statement that roots are the primary source of Hg in contaminated areas. (1) This is unpublished work and judging from the abstract it appears they state the atmosphere as the source not the roots; (2) This is at a former Hg mine – there is MASSIVE legacy emissions of Hg(0) to the atmosphere continuing to this day at these sites, which is readily available for stomatal assimilation; (3) as previously mentioned there is countless studies during the past 50 years that show root uptake in tree ubiquitously is an very minor, if not insignificant uptake pathway. This statements needs correcting. This statement is removed.
Lines 107-108: How does this compare to recommended soil guidelines? Please state this. This phrase is corrected as follows “Adding to that, soil samples collected from different areas in southern Lebanon showed values of Hg concentration ranging between 160-6480 ng/g showing a high contamination level as indicated by World reference Senesi et al. 1999, Kabata-Pendias 2001 ”.
Lines 144: I really don’t see the benefit of making acronyms of the sampling sites. Both a one-word towns and this just confuses readers that are not as closely linked to the study as the authors. I recommend simply writing the town names each time. We agreed and changed the acronyms to town names.
Figure 1: The climate graphs are really ancillary metadata. These are described in the method text and should be moved to the SI. Indeed, even the site map could be move to the supplementary information (SI). There are only two studies sites and again their location, climate and geography and surrounding Hg sources are described in the text. I believe this whole figure would be better served in a SI. On the behalf of all the authors, we believe it is better for the reader to have those sites indicated directly after the text to make it clearer concerning the locations. Concerning the climatic data, it is removed since it is available in the text and also indicated in figure 3.
Line 177: “8-15 m foot circumference”. There are obvious errors here. Also I highly recommend using diameter rather than circumference. It is much easier for the reader to comprehend. This sentence has been changed to: “For the Hg concentration analysis, four olive trees (3 to 5 m diameter and an average height of the trees 4-6m) were sampled in each of the two groves”
Line 196-197: While I do not think this is a major problem as I believe there will be minimal Hg(0) on surfaces or within foliage and stems, it needs to be acknowledge that this heated drying method would likely eliminated any and ALL Hg(0) present in the samples. This statement was amended to the following “Collected foliage and stems were rinsed with distilled water and then dried for 48 hours in an oven at a temperature of 50°C at maximum in order to remove any dust Hg from the surface of the samples following the method of Demers et al. 2013, Li et al. 2017, Pleijel et al. (2021). This procedure likely eliminate any Hg(0) present in the samples”.
Line 217: The detection limit should be listed as total mass of Hg, not concentration of Hg. The system does not analyse concentration (that is calculated by the mass of sample input), it is calibrated to determine the mass of Hg in any given sample. This is an important distinction. The calculation yielded the MDL of 0.04ng in units of mass (corresponding to a concentration of 0.7 ng g−1). This value was in the range of our measured Hg concentrations, which ranged from 0.4 ng for stem, 0.7 ng for soil and 0.8 ng for leaves and litter.
Line 261-263: What Hg concentration is being referred to here? Hg(0) Concentration in the air? This needs to be stated. This ambiguity is exactly why the results and discussion should be combined. The Hg concentration referred to here is the Hg content in the foliage and stems and it is the combination of Hg remaining in the foliage and stems after being rinsed and heated.
Lines 312: “the main Hg content” should be changed to “Highest Hg concentration”. This phrase is corrected as follows: “Our data corroborates previous studies (Bargagli 1995; Higueras et al. 2016; Naharro et al. 2018) showing that olive foliage has the highest Hg concentration of plant tissue.
Lines 318-322: Once again, I disagree the main source of Hg in the stems of the trees is from the soils. What is the evidence for this? I could reference 10+ papers that have shown Hg in woody materials of trees to be almost exclusively derived from foliage and downward transport in phloem. Higher concentrations in leaves over stems is NOT evidence that Hg in stems is derived from the roots. Not all Hg taken up by foliage is transferred to woody materials, which leads to an enrichment of Hg in foliage compared to Hg in woody materials. The phrase was amended to the following “Since normally the main source of Hg into the foliage is atmospheric and minimally through the soil, this explains the higher Hg concentration in the foliage, while in the stems the main uptake is from the foliage and transported through the phloem and soil, and it seems to be much lower levels of Hg concentration are found in the stems than that in the foliage (Pant et al., 2010; Tomiyasu et al., 2005).”
Lines 323-324: The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils. This phrase is added: “The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils.”
Lines 333-335: This concept of soil properties driving Hg concentrations and uptake in soils was not something introduced by O’Connor et al., 2019. This is not a new idea and again has been known of for decades. I agree, but I am giving here an updated reference.
Line 336: What does nitrogen content have to do with Hg sorption and uptake in soils? Nitrogen can also be a factor affecting the Hg content in soil depending on its characteristics. Nitrogen supply prevents oxidative stress in roots, but also can improve root development and increase the uptake of Hg from the soil (Carrasco-Gil et al. 2012).
Lines 339-341: Did the authors measure wet and dry deposition of Hg? For wet deposition to occur, there would need to be considerable Hg(II) in the atmosphere. I also see no reason as to why dry deposition would occur more in a higher temperature region. Higher temperatures favour partitioning of Hg(0) back into the gas phase, which would be suggestive of less dry deposition. The author discussed dry deposition and indicated the effect of higher temperature on higher dry deposition.
Lines 342-344: Foliage accumulates Hg(0) over time. Naturally older leaves that eventually die and senesce will be more enriched in Hg than younger leaves growing on the trees. I agree with your statement, that is why we indicated that older leaves have higher Hg concentration than younger leaves.
Lines 348-404 and Figure 3: This is far too speculative. These data are for Europe (the Authors use a site from Germany for Hg(0) in Figure 3). Lebanon is a long way from Europe and in a totally different climatic zone without typical northern temperate/boreal deciduous/conifer dominated forests. To make any sort of statement about atmospheric Hg zero concentrations this should have been measured or data taken from a long-term monitoring station in this climatic region/ecological biome. I see this whole (and very long discussion) on correlations between foliage and atmospheric Hg(0) to be too speculative to the point it is invalid. I also agree with the other reviewer that the emergence of foliage in olive trees in spring/early summer is the major driver here.
In the process of the reply to the review we rechecked the papers available on the atmospheric mercury closer to our region. We were not able to find any information in the region on atmospheric mercury data, but we passed across an interesting newly published paper by Martino et al. 2022 that is the only atmospheric gaseous elemental mercury (GEM) measurement near an olive site that is comparable to our study. Martino et al. 2022 publication shows the factors affecting the GEM values are mainly the factories, sea emissions, fires, and vegetation. Even though this paper is not in the same area as that of our sites, we take profit to compare with same years studied (2019-2020). We used their data from table 1 representing the monthly median GEM values and compared it to our data. Our values of the foliage in Bchaaleh and Kawkaba show higher values of Hg concentration in winter and lower in summer, were seasonality is clear the same as that of Martino et al. 2022. This seasonality can change due to many factors such as wind, fires, industrial surroundings and other factors. At the same time we decided to remove the northern Hemesphere data upon your feedback. We will change the discussion and narrow it to the Mediterranean region with the data mentioned in our comments.
Other data was done on Total Gaseous Mercury (TGM) in the French coastal Mediterranean site which showed higher values in winter/spring and lower values in summer in the Mediterranean region. This studies also showed seasonality with the atmospheric mercury due to dispersion of pollutants in the troposphere and high TGM values that are due to air masses from regional and local sources (Marusczak et al. 2015).
Mastromonaco et al. 2017 Showed also that the main source of mercury occurs from the Mediterranean Sea water and is affected by wind speed with higher values of GEM concentration registered in autumn in comparison to summer, with higher evasion rates found in summer in comparison to the autumn season. Another study that includes countries closer to Lebanon by Kotnik et al. 2014 found a seasonal variation in Hg in the Mediterranean region near Spain, Italy, Turkey and Israel (1998-1999) with the highest Hg average concentration during winter and the lowest during autumn.
Concerning the emergence of foliage in olive trees in spring/early summer being a major driver, this is shown through the seasonality registered during the different seasons due to the merging of three different years of the foliage in our study.
Lines 417-448: These paragraphs need grammatical and English language corrections. It is very hard to follow and from what I can derive it again seems highly speculative and to contradict the state of the science without data to support that. These paragraphs were checked by a scientific English editor. Some data was given to support our scientific point of view, but we tried to give our point of view in regards to the Hg cycling in stems, litter and soil system.
Citation: https://doi.org/10.5194/egusphere-2022-174-AC4
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AC4: 'Reply on AC3', Nagham Tabaja, 12 Oct 2022
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-174', Håkan Pleijel, 11 Aug 2022
This manuscript contains interesting new information, which should be of interest to the broader scientific audience. The Hg concentration of leaves, fruits, stems, litter of olive trees and soil (at different depths) in two geographically separated olive groves in Lebanon was investigated. Very little information on this is available in the scientific literature. Importantly the seasonal dynamics of the Hg concentrations of the different compartments were investigated. The relative importance of root uptake vs. uptake by leaves from the atmosphere is discussed.
I have two main concerns with the manuscript. Firstly, the language and clarity in several statements should be revised to improve the presentation. Please find below a number of suggestions of possible improvements. A native English speaker should check the entire manuscript. Secondly, the phenology of leaf dynamics in Olea europaea has been neglected in the interpretation of seasonal variation in leaf concentrations of Hg. According to literature a new flush of leaves emerges in spring in olive trees, while leaves are perennial and remain on the tree approximately 3 years. The new leaves that emerge and grow in spring and early summer have not been exposed to Hg to any large extent. Thus, they can be expected to start their development with very low concentrations of Hg – the Hg level increases rather monotonically with leaf age in many tree species (see Wohlegemuth et al 2021, Pleijel et al 2021). Thus, leaves sampled after the new flush of leaves has emerged and grown will typically have a lower Hg concentration than before the new leaves have emerged and become fully developed. Wouldn´t such a “dilution” effect explain (at least part of) the seasonal dynamics in foliage Hg concentration presented in Figure 2 a and b? If so, it would also influence the interpretation of the seasonal dynamics in the Discussion. The influence of the leaf phenology dynamics on foliage Hg concentration needs to be discussed to improve the quality of the manuscript.
Detailed comments:
Lines 21-22: clearer like this: “… to investigate the seasonality of the mercury (Hg) concentration of olive, an iconic tree …”.
Line 22: please change “was” to “were”.
Line 27: maybe “low” rather than “lowest” since stems seem to have had equally low or lower concentration (line 24).
Line 30: “global”? Rather it is relevant for the kind of vegetation typical of the Mediterranean basin and similar climates, which is in itself important.
Line 38: “most important” or “most widely distributed” rather than “most common”?
Line 51: maybe simplify – “Forests are known to act …”.
Line 52: rather “leaf gas exchange” than “photosynthesis”.
Line 54-55: maybe – “… or transferred to other plant organs …”.
Line 60: replace “is said” by “has been estimated”?
Line 64: “earth”? Do you mean “terrestrial”?
Line 68: is it really through “photo-respiration”? I think not. Please be more specific about the relevant “biological processes” referred to.
Line 69: “… Hg exchange between …”.
Line 72-73: The sentence starting “Differences …” is not clear.
Line 94: maybe clearer to say “an air pollution emission area”?
Line 111: “outcomes and consumed”? Not clear.
Lines 114-115: then sentence starting “In addition …” is not complete.
Line 117-119: the objectives would become more informative if they were further elaborated and detailed. The first one could include the information that leaves, fruits, stem, litter and soil were investigated (or this could be an additional, first objective – the comparison of the strongly contrasting Hg concentration levels between the different fractions is a very important aspect of the study). The second one is incomplete and should express that the significance of soil uptake is assessed in relation to uptake by the leaves of Hg from the atmosphere. The sites are locally uncontaminated but considering the scale of the Hg problem a certain degree of contamination occurs over wide areas.
Line 142: it does not become clear if Chekka town is a source of Hg emission or only of other pollutants.
Line 143: “monoxide” should be “carbon monoxide”? “particulate matter”.
Line 224: the sentence is not well phrased.
Line 247-249: why is not the concentrations of fruits included in these comparisons?
Line 266-267: the sentence starting “Seasonal effect …” is not complete.
Line 282 and section 3.3: it does not become completely clear what is compared. Variation among trees in different parts of the investigated areas? The presentation needs improvement in this section.
Line 298: if the p-value is 0.013 it is <0.05 and can thus be considered significant, contrary to what is said in the text!? Why is only one p-value provided if both foliage and stems are tested vs temperature?
Line 343: it could be discussed further why litter typically has higher concentrations than leaves. It should be mentioned here that the leaves shed as litter are likely to mostly be the oldest leaves, which have accumulated Hg during the longest period of time and thus have higher Hg concentrations than the remaining leaves have on average since they consist of both younger and older leaves. It should also be kept in mind that litterfall could have lost organic carbon, thereby concentrating Hg (e.g., Pokharel and Obrist, 2011).
Line 353: rather than “photosynthetic activity and stomatal conductance” it would be more accurate to say “accumulation in leaves after stomatal uptake”?
Line 440: since the distribution of Hg pollution is by nature geographically very widespread, long-distance contamination occurs and it may be better to say “In sites without local contamination” instead of “In uncontaminated sites”. Similarly on line 452 it would be appropriate to say “locally uncontaminated” rather than “non-contaminated”.
Line 459: “but it can also be due to the physiology” – what aspects of physiology? Photosynthetic activity and stomatal conductance already mentioned on line 458 are physiological processes! What other physiological processes do the authors refer to?
Line 467: “… between Hg soil due …”? Difficult to understand.
Why are some references in capitals, e.g., lines 512-513, lines 521-523, lines 712-715. Pleijel et al and Wohlgemuth et al are no longer preprints.
Font size in figures should be increased to improve readability.
In general, the results are interesting and important. They should be published, but as already mentioned the Hg concentration of foliage in relations to phenological dynamics of olive tree leaves needs to be considered in the discussion of the results.
Reference:
Pokharel, A. K. and Obrist, D.: Fate of mercury in tree litter during decomposition, Biogeosciences, 8, 2507–2521, https://doi.org/10.5194/bg-8-2507-2011, 2011.
Citation: https://doi.org/10.5194/egusphere-2022-174-RC1 -
AC1: 'Reply on RC1', Nagham Tabaja, 01 Oct 2022
We are very thankful for your constructive comments and feedback on the manuscript submitted. We accepted all the comments provided and amended them accordingly.
Concerning the language and clarity in several statements were revised and improved. A native English speaker checked the manuscript and helped in improving the form. In regards to the phenology of leaf dynamics, it has been indicated that the collected samples were merged from three different years and merged for analysis. In addition the physiology has been more integrated in the text but of course more detailed work can be made separetly to focus on this aspect.
Line 21-22: It is corrected.
Line 22: It is corrected.
Line 27: It is corrected by “It is noteworthy that olive fruits also have low Hg concentration (~7-11 ng/g).”
Line 30: It is corrected by “This may draw an adequate baseline for Eastern Mediterranean and region with similar climate inventories on Hg vegetation uptake and new studies on olive trees in the Mediterranean to reconstruct regional Hg pollution concentrations in the past and present.”
Line 38: It is corrected by “Mercury (Hg) is among the most widely distributed heavy metals polluting the Earth (Briffa et al. 2020).”
Line 51: It is corrected by “Forests are known to act as a sink of atmospheric Hg in the ecosystem.”
Line 52: It is corrected by “Plant foliage take up of Hg deposited on leaf surfaces through the stomata and leaf cuticles”.
Line 54-55: It is corrected by “where it accumulates with minimal mobility and small portions released back into the atmosphere or transferred to other plant organs”
Line 60: It is replaced from “is said” by “has been estimated”.
Line 64: It is replaced from “earth” by “terrestrial”.
Line 68: It is corrected by “Soil can release Hg to the atmosphere (Luo et al., 2016; Yang et al., 2018a; Assad, 2017; Schneider et al., 2019; Gworek et al., 2020; Pleijel et al., 2021) and also behave as a source of Hg to the plants.”
Line 69: It is corrected by “Trees are hence considered as important drivers of Hg exchange between the atmosphere and the soil (Yang et al. 2018) .”
Line 72-73: The sentence starting “Differences …” is not clear. This sentence is removed.
Line 94: It is corrected by “an air pollution emission area.”
Line 111: “outcomes and consumed”? Not clear. It is replaced by “for consumption.”
Lines 114-115: then sentence starting “In addition …” is not complete. This sentence is removed.
Line 117-119: The objectives are amended as per the feedback received. “In this study two sites, known for their century-old olive groves and located at two different altitudes in Lebanon, were selected to assess the Hg contents. In these remote areas, no direct sources of mercury contamination are reported and hence we expect very low Hg concentrations. However, due to atmospheric transport of Hg, deposition can be expected in remote areas (Grigal, 2003). The main objectives of this study are to examine and compare Hg levels in foliage, stems, fruits, litter and soil measured in each of these two olive groves, which we monitored monthly for 18 months. The second objective is to analyze the relative importance of Hg uptake from the soil to the foliage in comparison with the assimilation of atmospheric Hg by the leaves. Is the uptake of Hg from soil to foliage low in sites without local Hg contamination? Since the distribution of Hg pollution is by nature geographically widespread, and given the extent of Hg pollution in the Mediterranean and the transfer of pollution by wind and the Mediterranean Sea, long-distance contamination occurs over large areas. This study will fill the data gap on Hg pollution in the Eastern Mediterranean and may draw an adequate baseline for Eastern Mediterranean and region of similar climates inventories on Hg vegetation uptake and new studies on olive trees in the Mediterranean to reconstruct regional Hg pollution concentrations in the past and present.”
Line 142: it does not become clear if Chekka town is a source of Hg emission or only of other pollutants. This sentence is cleared by “To our knowledge no direct Hg pollution is reported at Chekka and Selaata sites.”.
Line 143: It is corrected by “carbon monoxide”.
Line 224: It is corrected by “For the statistical analysis we used the R 4.1.0 program. Our data are not normally distributed, so for the effect of tissue type on Hg concentration, Wilcoxon test was used with the tissue type (foliage and stems) as the main effect.”
Line 247-249: why is not the concentrations of fruits included in these comparisons? The fruits are added.
Line 266-267: It is corrected by “A seasonal effect on foliage and stems was registered”.
Line 282 and section 3.3: It is rearranged and corrected by “3.3. Tree comparison
In the upper terrace of BC grove, the foliage average Hg concentration of BCO4 and BCO1 varied between 42.4 ± 11.5 ng/g and 44.6 ± 13.3 ng/g respectively showing no significant difference (p-value = 0.8225). In the lower terrace of the same site, foliage average Hg concentrations of trees BCO12 and BCO9 were found to vary from 45.6 ± 12.7 ng/g to 60.7 ± 12.7 ng/g respectively) (Figure 2a,c). Significant differences are shown between foliage of the tree BCO9 and BCO1 (p-value= 0.0019), BCO4 and BCO12, p-value=0.00047) respectively).
In the upper terrace of BC grove, the stems average Hg concentration of BCO4 and BCO1 varied between 7.0 ± 2.8 ng/g and 7.1 ± 2.9 ng/g respectively showing no significant difference (p-value= 0.94). In the lower terrace, stems average Hg concentrations of BCO12 and BCO9 are 6.4 ± 2.2 ng/g nd 11.2 ± 5.2 ng/g respectively showing a significant difference of p-value= 0.0054 (Figure 2a,c). For BCO1 and BCO12 there was no significance difference registered with p-Value= 0.5725, the same goea for BCO4 and BCO12 no significance difference was registered with p-value= 0.523.
The average concentration per tree in foliage and stems were 32.4 ± 12.2 ng/g and 8.5 ± 4.0 ng/g respectively for KWO1, 32. 8 ± 14.7 ng/g and 8.9 ± 6.0 ng/g for KWO2, 37.6 ± 14.0 ng/g and 9.3 ± 6.7 ng/g for KWO3 and 37.7 ± 13.6 ng/g and 9.6 ± 4.0 ng/g for KWO4 (Figure 2b,d). In KW grove, comparison of the foliage Hg concentration between the four studied trees shows no significant difference (0.22<p-value<1) so did the stems (0.21<p-value<0.96).
In BC grove, the trees located on the lower terrace recorded higher Hg concentration values than those of the upper terrace especially tree BCO9. While KW grove had similar Hg concentration among all four trees.”
Line 298: It is corrected and values of foliage and stems are separated.
Line 343: It is amended by “In parallel, the litter showed higher Hg concentration than that in foliage in both BC (62.9 ± 17.8 ng/g) and KW (75.7 ± 20.3 ng/g) (Table 1) likely explained by the process of the Hg input into the litter through foliage shedding its oldest foliage to the forest floor which have accumulated Hg during the longest period of time and thus have higher Hg concentrations than the remaining foliage have on average since they consist of both younger and older foliage (Rea et al. 1996; Pleijel et al. 2021).”
Line 353: It is corrected by “accumulation in leaves after stomatal uptake.”
Line 440: since the distribution of Hg pollution is by nature geographically very widespread, long-distance contamination occurs and it may be better to say “In sites without local contamination” instead of “In uncontaminated sites”. Similarly on line 452 it would be appropriate to say “locally uncontaminated” rather than “non-contaminated”. It is corrected,
Why are some references in capitals, e.g., lines 512-513, lines 521-523, lines 712-715. Pleijel et al and Wohlgemuth et al are no longer preprints. Those references are corrected.
Font size in figures should be increased to improve readability. It is amended.
Citation: https://doi.org/10.5194/egusphere-2022-174-AC1
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AC1: 'Reply on RC1', Nagham Tabaja, 01 Oct 2022
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RC2: 'Comment on egusphere-2022-174', Anonymous Referee #4, 25 Aug 2022
This data set explores Hg uptake and cycling in olive groves – an critical agricultural tree across much of the Mediterranean basin. The experimental design seems valid to conduct an initial examination of Hg physiological cycling within olive trees and their surrounding soils. Additionally, the QA/QC of the data seems valid, which provides merit to at least the data.
Nonetheless, I cannot support publication of this manuscript in its current form for the reasons listed below. My recommendations would be to considerably scale back the scope of this manuscript and limit the discussion to the bounds of the data (extravagant speculations based on distant and unrelated data need to be removed), work on the grammar and English language of the paper, and consider the structural suggestions made below. At this point the paper may be worthy of resubmission and publication. Whether that publication is worth of publication in EGU-Sphere is a decision for the editor(s).
General comments:
- I feel like this paper would greatly benefit from combining the results and discussion. The current separated results and discussion lead to very dry reading that makes engagement somewhat difficult and there is clear repetition in the discussion section on what has already been written in the results. List then data and discuss it along in context of the story the manuscript is attempting to make. Additionally, there is repetition in the listing of the data between the text, figure 2 and table 2 (and the discussion). These data need only be listed one time.
- There is a lot of speculation and much of this speculation appears unsupported or poorly supported by the available data; the discussion oversteps the suggestions and conclusions that can be made with this dataset via the excessive speculation. (a) There is a whole section looking at correlations between atmospheric Hg(0) from Europe (specifically it would seems Schauinsland, Germany; And data from 7 years prior) and foliage concentrations in these Lebanese olive groves, which are completely different ecological biomes and climatic zones. The spatiotemporal differences between these datasets is vast. (b) The authors jump between soils being a source of Hg to aerial parts of the plant and back to the atmosphere being the source without any definitive evidence that can clearly define which source is the actual source. Suggestions of soils sources would also contradict basically all (and a very large pool of) evidence that suggests this uptake pathway is minimal in trees. If the authors want to suggest this is a significant pathway they need strong evidence to do so and this study does not provide such evidence.
- I also believe the suggestion by another review that new foliage emergence is the main driver of the observed seasonal differences in foliar mercury is valid and needs detailed consideration.
Line 38-39: The term heavy metals is a poor descriptor and one that has been suggested multiple times to be made redundant (Duffus, 2009: https://doi.org/10.1515/ci.2001.23.6.163; Pourrett and Hursthouse, 2019: 10.3390/ijerph16224446 ). I would suggest changing the terminology throughout with a less ambiguous descriptor like “potentially toxic metals”.
Lines 44-45: change to “Hg(0) is primarily transferred through the atmosphere by air mass movement and can undergo long-range transport”.
Lines 45-47: This is incorrect as written. Hg(0) does not “covalently bond with organic groups to forming… MeHg”. It must first be oxidized (either in the atmosphere or in terrestrial matrices after deposition), transferred to anoxic or poorly oxic conditions and it can then be methylated.
Lines 47-48: These descriptions about legacy mercury are extremely vague and need to be improved. It is also a bit out of place with the rest of the story and I think these two sentences could be deleted without effect.
Lines 48-50: Needs grammatical correction.
Lines 51: Delete “in the ecosystem”.
Lines 51-63: This paragraph needs grammatical and structural (and English language) work. It is a bit disjointed and jumps from one thought to another continuously.
Lines 67-69: This ignores one of the most critical fluxes of Hg back to the atmosphere from forests: wildfires. Please add a statement on this and include references such as: McLagan et al. (2021) 10.5194/acp-638 21-5635-2021; Dastoor et al. (2022) 10.1038/s43017-022-00269-w; Friedli et al. (2009) 10.1021/es802703g.
Lines 73-76: I disagree with this statement. Tree ring Hg (dendrochronology) is predominantly used as an archiving tool for atmospheric Hg(0) (Hg(0) oxidised in leaves, transferred in phloem to bole wood, and generally considered to be stored long-term). It has been established for decades (Beauford et al., 1977: 0.1111/j.1399-3054.1977.tb01880.x; Lindberg et al. (1979) 10.2134/jeq1979.00472425000800040026x) and re-confirmed many times since that Hg in woody materials is derived from atmosphere. Please correct these statements accordingly.
Lines 76-77: I would suggest to add McLagan et al., (2022: https://doi.org/10.5194/bg-2022-124, recently accepted) to this reference on Hg dendrochemistry (using stable Hg isotopes). Some of the findings in this recently accepted study may be highly beneficial to this manuscript.
Lines 102-103: I cannot agree with this statement that roots are the primary source of Hg in contaminated areas. (1) This is unpublished work and judging from the abstract it appears they state the atmosphere as the source not the roots; (2) This is at a former Hg mine – there is MASSIVE legacy emissions of Hg(0) to the atmosphere continuing to this day at these sites, which is readily available for stomatal assimilation; (3) as previously mentioned there is countless studies during the past 50 years that show root uptake in tree ubiquitously is an very minor, if not insignificant uptake pathway. This statements needs correcting.
Lines 107-108: How does this compare to recommended soil guidelines? Please state this.
Lines 144: I really don’t see the benefit of making acronyms of the sampling sites. Both a one-word towns and this just confuses readers that are not as closely linked to the study as the authors. I recommend simply writing the town names each time.
Figure 1: The climate graphs are really ancillary metadata. These are described in the method text and should be moved to the SI. Indeed, even the site map could be move to the supplementary information (SI). There are only two studies sites and again their location, climate and geography and surrounding Hg sources are described in the text. I believe this whole figure would be better served in a SI.
Line 177: “8-15 m foot circumference”. There are obvious errors here. Also I highly recommend using diameter rather than circumference. It is much easier for the reader to comprehend.
Line 196-197: While I do not think this is a major problem as I believe there will be minimal Hg(0) on surfaces or within foliage and stems, it needs to be acknowledge that this heated drying method would likely eliminated any and ALL Hg(0) present in the samples.
Line 217: The detection limit should be listed as total mass of Hg, not concentration of Hg. The system does not analyze concentration (that is calculated by the mass of sample input), it is calibrated to determine the mass of Hg in any given sample. This is an important distinction.
Line 261-263: What Hg concentration is being referred to here? Hg(0) Concentration in the air? This needs to be stated. This ambiguity is exactly why the results and discussion should be combined.
Lines 312: “the main Hg content” should be changed to “Highest Hg concentration”.
Lines 318-322: Once again, I disagree the main source of Hg in the stems of the trees is from the soils. What is the evidence for this? I could reference 10+ papers that have shown Hg in woody materials of trees to be almost exclusively derived from foliage and downward transport in phloem. Higher concentrations in leaves over stems is NOT evidence that Hg in stems is derived from the roots. Not all Hg taken up by foliage is transferred to woody materials, which leads to an enrichment of Hg in foliage compared to Hg in woody materials.
Lines 323-324: The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils.
Lines 333-335: This concept of soil properties driving Hg concentrations and uptake in soils was not something introduced by O’Connor et al., 2019. This is not a new idea and again has been known of for decades.
Line 336: What does nitrogen content have to do with Hg sorption and uptake in soils?
Lines 339-341: Did the authors measure wet and dry deposition of Hg? For wet deposition to occur, there would need to be considerable Hg(II) in the atmosphere. I also see no reason as to why dry deposition would occur more in a higher temperature region. Higher temperatures favour partitioning of Hg(0) back into the gas phase, which would be suggestive of less dry deposition.
Lines 342-344: Foliage accumulates Hg(0) over time. Naturally older leaves that eventually die and senesce will be more enriched in Hg than younger leaves growing on the trees.
Lines 348-404 and Figure 3: This is far too speculative. These data are for Europe (the Authors use a site from Germany for Hg(0) in Figure 3). Lebanon is a long way from Europe and in a totally different climatic zone without typical northern temperate/boreal deciduous/conifer dominated forests. To make any sort of statement about atmospheric Hg zero concentrations this should have been measured or data taken from a long-term monitoring station in this climatic region/ecological biome. I see this whole (and very long discussion) on correlations between foliage and atmospheric Hg(0) to be too speculative to the point it is invalid. I also agree with the other reviewer that the emergence of foliage in olive trees in spring/early summer is the major driver here.
Lines 417-448: These paragraphs need grammatical and English language corrections. It is very hard to follow and from what I can derive it again seems highly speculative and to contradict the state of the science without data to support that.
Citation: https://doi.org/10.5194/egusphere-2022-174-RC2 -
AC2: 'Reply on RC2', Nagham Tabaja, 02 Oct 2022
Your input to this study is much appreciated and of great help to this work in order to improve it and make it better for publication. Your recommendations were taken into account and the English and grammar were worked on by the help of a scientific English editor.
In this paper we did not combine the results and discussion, following the journal way of dividing the different parts of the article. The repetition that can be found in the discussion is minimal and is to make it easier for the reader to relate more directly without having to go back to the results part that is much smaller in comparison to the discussion.
The repetition between the text and figure and the table is to show the needed information in a more visual way and in a form of values for the reader to be able to be in contact with the reality of the study.
The speculations made are based on the few data found and on our own analysis and point of view on the scientific data. The comparison made with the Northern Hemisphere from data of other countries far from our region is due to the lack of data in the Easter Mediterranean. The main suggestion of the source of the Hg concentration is the atmosphere and a minimal source from the soil that does need more study and dedication.
In this study three different generations of foliage were mixed and studied that may have affected the seasonality in foliar mercury.
Line 38-39: The term heavy metals is a poor descriptor and one that has been suggested multiple times to be made redundant (Duffus, 2009: https://doi.org/10.1515/ci.2001.23.6.163; Pourrett and Hursthouse, 2019: 10.3390/ijerph16224446 ). I would suggest changing the terminology throughout with a less ambiguous descriptor like “potentially toxic metals”. This phrase is corrected.
Lines 44-45: change to “Hg(0) is primarily transferred through the atmosphere by air mass movement and can undergo long-range transport”. This phrase is corrected as suggested.
Lines 45-47: This is incorrect as written. Hg(0) does not “covalently bond with organic groups to forming… MeHg”. It must first be oxidized (either in the atmosphere or in terrestrial matrices after deposition), transferred to anoxic or poorly oxic conditions and it can then be methylated. It is corrected by “This highly diffusive Hgcan easily pass biological barriers (i.e. cell membranes, foliage, skin) and it must first be oxidized (either in the atmosphere or in terrestrial matrices after deposition), transferred to anoxic or poorly oxic conditions and bind covalently with organic groups forming the widespread toxic methylmercury (MeHg, CH3Hg+) (Clarkson and Magos 2006).”
Lines 47-48: These descriptions about legacy mercury are extremely vague and need to be improved. It is also a bit out of place with the rest of the story and I think these two sentences could be deleted without effect. These sentences were deleted.
Lines 48-50: Needs grammatical correction. It was revised by an English scientific editor.
Lines 51: Delete “in the ecosystem”. This word is deleted.
Lines 51-63: This paragraph needs grammatical and structural (and English language) work. It is a bit disjointed and jumps from one thought to another continuously. This paragraph was checked and revised by an English scientific editor.
Lines 67-69: This ignores one of the most critical fluxes of Hg back to the atmosphere from forests: wildfires. Please add a statement on this and include references such as: McLagan et al. (2021) 10.5194/acp-638 21-5635-2021; Dastoor et al. (2022) 10.1038/s43017-022-00269-w; Friedli et al. (2009) 10.1021/es802703g. This phrase is added “An important source of Hg emission to the atmosphere is from the biomass burning, ocean currents and rivers and is also considered an essential component of the Hg global biochemical cycle (Friedli et al., 2009; McLagan et al., 2021; Dastoor et al., 2022).”
Lines 73-76: I disagree with this statement. Tree ring Hg (dendrochronology) is predominantly used as an archiving tool for atmospheric Hg(0) (Hg(0) oxidised in leaves, transferred in phloem to bole wood, and generally considered to be stored long-term). It has been established for decades (Beauford et al., 1977: 0.1111/j.1399-3054.1977.tb01880.x; Lindberg et al. (1979) 10.2134/jeq1979.00472425000800040026x) and re-confirmed many times since that Hg in woody materials is derived from atmosphere. Please correct these statements accordingly. I used this statement from a new reference from Yanai et al. 2020.
Lines 76-77: I would suggest to add McLagan et al., (2022: https://doi.org/10.5194/bg-2022-124, recently accepted) to this reference on Hg dendrochemistry (using stable Hg isotopes). Some of the findings in this recently accepted study may be highly beneficial to this manuscript. The following phrase is added “Another study that used Hg stable isotopes propose a reemission and reduction of the foliage Hg can also take place of this internal leaf Hg (between 29 and 42 % of gross uptake based on the plant species studied) may occur (Yuan et al., 2018)”.
Lines 102-103: I cannot agree with this statement that roots are the primary source of Hg in contaminated areas. (1) This is unpublished work and judging from the abstract it appears they state the atmosphere as the source not the roots; (2) This is at a former Hg mine – there is MASSIVE legacy emissions of Hg(0) to the atmosphere continuing to this day at these sites, which is readily available for stomatal assimilation; (3) as previously mentioned there is countless studies during the past 50 years that show root uptake in tree ubiquitously is an very minor, if not insignificant uptake pathway. This statements needs correcting. This statement is removed.
Lines 107-108: How does this compare to recommended soil guidelines? Please state this. This phrase is corrected as follows “Adding to that, soil samples collected from different areas in southern Lebanon showed values of Hg concentration ranging between 160-6480 ng/g showing a high contamination levels”.
Lines 144: I really don’t see the benefit of making acronyms of the sampling sites. Both a one-word towns and this just confuses readers that are not as closely linked to the study as the authors. I recommend simply writing the town names each time. This acronomy was made to make it easier on readers to distinguish between sites without having to spell them each time over the study knowing how hard it is to pronounce them.
Figure 1: The climate graphs are really ancillary metadata. These are described in the method text and should be moved to the SI. Indeed, even the site map could be move to the supplementary information (SI). There are only two studies sites and again their location, climate and geography and surrounding Hg sources are described in the text. I believe this whole figure would be better served in a SI. In my opinion i believe it is better for the reader to have those sites indicated directly after the text to make it clearer concerning the locations.
Line 177: “8-15 m foot circumference”. There are obvious errors here. Also I highly recommend using diameter rather than circumference. It is much easier for the reader to comprehend. These data were rechecked and there is no error in the values.
Line 196-197: While I do not think this is a major problem as I believe there will be minimal Hg(0) on surfaces or within foliage and stems, it needs to be acknowledge that this heated drying method would likely eliminated any and ALL Hg(0) present in the samples. This statement was amended to the following “Collected foliage and stems were rinsed with distilled water and then dried for 48 hours in an oven at a temperature of 60°C at maximum in order to remove any dust Hg from the surface of the samples.”
Line 217: The detection limit should be listed as total mass of Hg, not concentration of Hg. The system does not analyse concentration (that is calculated by the mass of sample input), it is calibrated to determine the mass of Hg in any given sample. This is an important distinction. The detection limit is changed to total mass as suggested.
Line 261-263: What Hg concentration is being referred to here? Hg(0) Concentration in the air? This needs to be stated. This ambiguity is exactly why the results and discussion should be combined. The Hg(0) is the referred to Hg concentration.
Lines 312: “the main Hg content” should be changed to “Highest Hg concentration”. This phrase is corrected.
Lines 318-322: Once again, I disagree the main source of Hg in the stems of the trees is from the soils. What is the evidence for this? I could reference 10+ papers that have shown Hg in woody materials of trees to be almost exclusively derived from foliage and downward transport in phloem. Higher concentrations in leaves over stems is NOT evidence that Hg in stems is derived from the roots. Not all Hg taken up by foliage is transferred to woody materials, which leads to an enrichment of Hg in foliage compared to Hg in woody materials. I agree with your argument, but as said minima Hg(0) is transferred from foliage to stems and not all comes from the roots .
Lines 323-324: The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils. This phrase is added : The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils.”
Lines 333-335: This concept of soil properties driving Hg concentrations and uptake in soils was not something introduced by O’Connor et al., 2019. This is not a new idea and again has been known of for decades. I agree, but I am giving here an updated reference.
Line 336: What does nitrogen content have to do with Hg sorption and uptake in soils? Nitrogen can also be a factor affecting the Hg content in soil depending on its characteristics.
Lines 339-341: Did the authors measure wet and dry deposition of Hg? For wet deposition to occur, there would need to be considerable Hg(II) in the atmosphere. I also see no reason as to why dry deposition would occur more in a higher temperature region. Higher temperatures favour partitioning of Hg(0) back into the gas phase, which would be suggestive of less dry deposition. The author discussed dry deposition and indicated the effect of higher temperature on higher dry deposition.
Lines 342-344: Foliage accumulates Hg(0) over time. Naturally older leaves that eventually die and senesce will be more enriched in Hg than younger leaves growing on the trees. I agree with your statement.
Lines 348-404 and Figure 3: This is far too speculative. These data are for Europe (the Authors use a site from Germany for Hg(0) in Figure 3). Lebanon is a long way from Europe and in a totally different climatic zone without typical northern temperate/boreal deciduous/conifer dominated forests. To make any sort of statement about atmospheric Hg zero concentrations this should have been measured or data taken from a long-term monitoring station in this climatic region/ecological biome. I see this whole (and very long discussion) on correlations between foliage and atmospheric Hg(0) to be too speculative to the point it is invalid. I also agree with the other reviewer that the emergence of foliage in olive trees in spring/early summer is the major driver here. Another data done on the western Mediterranean showed the same results as we implicated in our study (Mastromonaco et al. 2017). Concerning this part, the problem is that there is no data related to the Eastern Mediterranean that we could find to use as a comparison in our study. But this can be a starting point to have more data collection that is closer to our studied regions.
Lines 417-448: These paragraphs need grammatical and English language corrections. It is very hard to follow and from what I can derive it again seems highly speculative and to contradict the state of the science without data to support that. These paragraphs were checked by a scientific English editor. Some data was given to support our speculations, but we tried to give our point of view in regards to the Hg cycling n stems, litter and soil system.
Citation: https://doi.org/10.5194/egusphere-2022-174-AC2 -
AC3: 'Reply on RC2', Nagham Tabaja, 11 Oct 2022
Please disregard the first response and check the second response to the comments given.
Response to the comment on EGUsphere-2022-174 by Hakan Pleijel (Referee 1) on the article “ Seasonal variation of mercury concentration of ancient olive groves of Lebanon” by Tabaja et al. https://doi.org/10.5194/egusphere-2022-174-RC1, 2022
We are very thankful for your constructive comments and feedback on the manuscript submitted. We accepted all the comments provided and amended them accordingly.
Concerning the language and clarity in several statements were revised and improved. A native English speaker checked the manuscript and helped in improving the form. In regards to the phenology of leaf dynamics, it has been indicated that the collected samples were merged from three different years and merged for analysis. The phenological growth stages of olive trees described by Sanz-Cortès et al. (2002) in Spain suggest leaf development from March to November. Hence the Hg concentration measured on monthly collected leaves represents an average of Hg accumulated in young leaves (year N of collection) and older leaves (N-1 year and N-2 years) where N is equal to 2019 and 2020. According to Pleijel et al. 2021, young leave has less mercury than older leaves generations. We can expect that our Hg concentration is lower during the growth of young leaves due to the Hg dilution signal with the low Hg content of the younger leaves. In a very simple approach, we can suggest the late winter-early spring should exhibit the lowest Hg contents. However, our results show the lowest values in summer when young leaves have already grown significantly. In addition, physiology has been more integrated into the text but of course, more detailed work can be made separately to focus on this aspect.
Line 21-22: It is corrected.
Line 22: It is corrected.
Line 27: It is corrected by “It is noteworthy that olive fruits also have low Hg concentration (~7-11 ng/g).”
Line 30: It is corrected by “This may draw an adequate baseline for Eastern Mediterranean and region with similar climate inventories on Hg vegetation uptake and new studies on olive trees in the Mediterranean to reconstruct regional Hg pollution concentrations in the past and present.”
Line 38: It is corrected by “Mercury (Hg) is among the most widely distributed heavy metals polluting the Earth (Briffa et al. 2020).”
Line 51: It is corrected by “Forests are known to act as a sink of atmospheric Hg in the ecosystem.”
Line 52: It is corrected by “Plant foliage take up of Hg deposited on leaf surfaces through the stomata and leaf cuticles”.
Line 54-55: It is corrected by “where it accumulates with minimal mobility and small portions released back into the atmosphere or transferred to other plant organs”
Line 60: It is replaced from “is said” by “has been estimated”.
Line 64: It is replaced from “earth” by “terrestrial”.
Line 68: It is corrected by “Soil can release Hg to the atmosphere (Luo et al., 2016; Yang et al., 2018a; Assad, 2017; Schneider et al., 2019; Gworek et al., 2020; Pleijel et al., 2021) and also behave as a source of Hg to the plants.”
Line 69: It is corrected by “Trees are hence considered as important drivers of Hg exchange between the atmosphere and the soil (Yang et al. 2018).”
Line 72-73: The sentence starting “Differences …” is not clear. This sentence is removed.
Line 94: It is corrected by “an air pollution emission area.”
Line 111: “outcomes and consumed”? Not clear. It is replaced by “for consumption.”
Lines 114-115: then sentence starting “In addition …” is not complete. This sentence is removed.
Line 117-119: The objectives are amended as per the feedback received. “In this study two sites, known for their century-old olive groves and located at two different altitudes in Lebanon, were selected to assess the Hg contents. In these remote areas, no direct sources of mercury contamination are reported and hence we expect very low Hg concentrations. However, due to atmospheric transport of Hg, deposition can be expected in remote areas (Grigal, 2003). The main objectives of this study are to examine and compare Hg levels in foliage, stems, fruits, litter and soil measured in each of these two olive groves, which we monitored monthly for 18 months. The second objective is to analyze the relative importance of Hg uptake from the soil to the foliage in comparison with the assimilation of atmospheric Hg by the leaves. Is the uptake of Hg from soil to foliage low in sites without local Hg contamination? Since the distribution of Hg pollution is by nature geographically widespread, and given the extent of Hg pollution in the Mediterranean and the transfer of pollution by wind and the Mediterranean Sea, long-distance contamination occurs over large areas. This study will fill the data gap on Hg pollution in the Eastern Mediterranean and may draw an adequate baseline for Eastern Mediterranean and region of similar climates inventories on Hg vegetation uptake and new studies on olive trees in the Mediterranean to reconstruct regional Hg pollution concentrations in the past and present.”
Line 142: it does not become clear if Chekka town is a source of Hg emission or only of other pollutants. This sentence is cleared by “To our knowledge no direct Hg pollution is reported at Chekka and Selaata sites.”.
Line 143: It is corrected by “carbon monoxide”.
Line 224: It is corrected by “For the statistical analysis we used the R 4.1.0 program. Our data are not normally distributed, so for the effect of tissue type on Hg concentration, Wilcoxon test was used with the tissue type (foliage and stems) as the main effect.”
Line 247-249: why is not the concentrations of fruits included in these comparisons? The fruits are added.
Line 266-267: It is corrected by “A seasonal effect on foliage and stems was registered”.
Line 282 and section 3.3: It is rearranged and corrected by “3.3. Tree comparison
In the upper terrace of BC grove, the foliage average Hg concentration of BCO4 and BCO1 varied between 42.4 ± 11.5 ng/g and 44.6 ± 13.3 ng/g respectively showing no significant difference (p-value = 0.8225). In the lower terrace of the same site, foliage average Hg concentrations of trees BCO12 and BCO9 were found to vary from 45.6 ± 12.7 ng/g to 60.7 ± 12.7 ng/g respectively) (Figure 2a,c). Significant differences are shown between foliage of the tree BCO9 and BCO1 (p-value= 0.0019), BCO4 and BCO12, p-value=0.00047) respectively).
In the upper terrace of BC grove, the stems average Hg concentration of BCO4 and BCO1 varied between 7.0 ± 2.8 ng/g and 7.1 ± 2.9 ng/g respectively showing no significant difference (p-value= 0.94). In the lower terrace, stems average Hg concentrations of BCO12 and BCO9 are 6.4 ± 2.2 ng/g nd 11.2 ± 5.2 ng/g respectively showing a significant difference of p-value= 0.0054 (Figure 2a,c). For BCO1 and BCO12 there was no significance difference registered with p-Value= 0.5725, the same goea for BCO4 and BCO12 no significance difference was registered with p-value= 0.523.
The average concentration per tree in foliage and stems were 32.4 ± 12.2 ng/g and 8.5 ± 4.0 ng/g respectively for KWO1, 32. 8 ± 14.7 ng/g and 8.9 ± 6.0 ng/g for KWO2, 37.6 ± 14.0 ng/g and 9.3 ± 6.7 ng/g for KWO3 and 37.7 ± 13.6 ng/g and 9.6 ± 4.0 ng/g for KWO4 (Figure 2b,d). In KW grove, comparison of the foliage Hg concentration between the four studied trees shows no significant difference (0.22<p-value<1) so did the stems (0.21<p-value<0.96).
In BC grove, the trees located on the lower terrace recorded higher Hg concentration values than those of the upper terrace especially tree BCO9. While KW grove had similar Hg concentration among all four trees.”
Line 298: It is corrected and values of foliage and stems are separated.
Line 343: It is amended by “In parallel, the litter showed higher Hg concentration than that in foliage in both BC (62.9 ± 17.8 ng/g) and KW (75.7 ± 20.3 ng/g) (Table 1) likely explained by the process of the Hg input into the litter through foliage shedding its oldest foliage to the forest floor which have accumulated Hg during the longest period of time and thus have higher Hg concentrations than the remaining foliage have on average since they consist of both younger and older foliage (Rea et al. 1996; Pleijel et al. 2021).”
Line 353: It is corrected by “accumulation in leaves after stomatal uptake.”
Line 440: since the distribution of Hg pollution is by nature geographically very widespread, long-distance contamination occurs and it may be better to say “In sites without local contamination” instead of “In uncontaminated sites”. Similarly on line 452 it would be appropriate to say “locally uncontaminated” rather than “non-contaminated”. It is corrected.
Why are some references in capitals, e.g., lines 512-513, lines 521-523, lines 712-715. Pleijel et al and Wohlgemuth et al are no longer preprints. Those references are corrected.
Font size in figures should be increased to improve readability. It is amended.
Citation: https://doi.org/10.5194/egusphere-2022-174-AC3 -
AC4: 'Reply on AC3', Nagham Tabaja, 12 Oct 2022
Please disregard the previous response and check instead the below.
Response to the comment on EGUsphere-2022-174 by Anonymous Referee on the article “ Seasonal variation of mercury concentration of ancient olive groves of Lebanon” by Tabaja et al. https://doi.org/10.5194/egusphere-2022-174-RC1, 2022
Thank you very much for the pertinent revision of our manuscript. Your input to this study is much appreciated and of great help to this work in order to improve it and make it better for publication. Your recommendations were taken into account and the English and grammar were revised by the help of a scientific English editor.
In this paper we did not combine the results and discussion, following the journal mostly used way of dividing the different parts of the article. The repetition that can be found in the discussion is minimal and is to make it easier for the reader to relate more directly without having to go back to the results part which is much smaller in comparison to the discussion.
The repetition between the text and figure and the table have been minimized.
The speculations made have been revised. We removed the comparison made with data of other countries far from our region. In our study, the main suggestion of the source of the Hg concentration is the atmosphere while only a minimal source from the soil was considered as reflected in the manuscript.
In this study three different generations of foliage were mixed and studied. We are aware that this certainly affected the seasonality in foliar mercury. This has been discussed following the rev 1 comments. We included in the text “For each olive tree, both sun exposed and shaded foliage (olive tree bears foliage from three different years -age classes current+2 (C+2) was collected during the same year) and stems (terminal portions of 20 cm) with no evidence of pathogens were randomly taken and merged from the upper, middle, and lower canopy position of the olive trees on a monthly basis using a manual pruner.”
Line 38-39: The term heavy metals is a poor descriptor and one that has been suggested multiple times to be made redundant (Duffus, 2009: https://doi.org/10.1515/ci.2001.23.6.163; Pourrett and Hursthouse, 2019: 10.3390/ijerph16224446 ). I would suggest changing the terminology throughout with a less ambiguous descriptor like “potentially toxic metals”. This phrase is corrected as follows: “Mercury (Hg) is among the most widely distributed potentially toxic metals polluting the Earth (Briffa et al. 2020)”.
Lines 44-45: change to “Hg(0) is primarily transferred through the atmosphere by air mass movement and can undergo long-range transport”. This phrase is corrected as suggested: “Hg(0) is primarily transferred through the atmosphere by air mass movement and can undergo long-range transport”.
Lines 45-47: This is incorrect as written. Hg(0) does not “covalently bond with organic groups to forming… MeHg”. It must first be oxidized (either in the atmosphere or in terrestrial matrices after deposition), transferred to anoxic or poorly oxic conditions and it can then be methylated. It is corrected by “Because of its high volatility and susceptibility to oxidation, elemental Hg(0) is the predominant form of Hg in the atmosphere. This highly diffusive Hg can easily pass biological barriers (i.e. cell membranes, foliage, skin). Mercury has three oxidation states, namely, Hg(0) (elemental mercury), Hg(I) (mercurous), or Hg(II) (mercuric), although Hg(I) mercurous form is not stable under typical environmental conditions and, therefore, is rarely observed. It is likely that the Hg(II) high binding affinities bind covalently with organic groups to forming the widespread toxic methylmercury (MeHg, CH3Hg+) (Du and Fang, 1983; Clarkson and Magos 2006; Pleijel et al., 2021).”
Lines 47-48: These descriptions about legacy mercury are extremely vague and need to be improved. It is also a bit out of place with the rest of the story and I think these two sentences could be deleted without effect. These sentences were deleted.
Lines 48-50: Needs grammatical correction. It was revised by an English scientific editor.
Lines 51:Delete “in the ecosystem”. This word is deleted.
Lines 51-63: This paragraph needs grammatical and structural (and English language) work. It is a bit disjointed and jumps from one thought to another continuously. This paragraph was checked and revised by an English scientific editor.
Lines 67-69: This ignores one of the most critical fluxes of Hg back to the atmosphere from forests: wildfires. Please add a statement on this and include references such as: McLagan et al. (2021) 10.5194/acp-638 21-5635-2021; Dastoor et al. (2022) 10.1038/s43017-022-00269-w; Friedli et al. (2009) 10.1021/es802703g. This phrase is added “Though variable from year to year, Hg emission to the atmosphere from biomass burning is considered as an important driver of the global Hg biogeochemical cycle ( Friedli et al., 2009; De Simone et al., 2015; McLagan et al., 2021; Dastoor et al., 2022)”.
Lines 73-76: I disagree with this statement. Tree ring Hg (dendrochronology) is predominantly used as an archiving tool for atmospheric Hg(0) (Hg(0) oxidised in leaves, transferred in phloem to bole wood, and generally considered to be stored long-term). It has been established for decades (Beauford et al., 1977: 0.1111/j.1399-3054.1977.tb01880.x; Lindberg et al. (1979) 10.2134/jeq1979.00472425000800040026x) and re-confirmed many times since that Hg in woody materials is derived from atmosphere. Please correct these statements accordingly. The studies of the Hg cycle in forest ecosystems show that gaseous elemental Hg(0) is the main source taken up by plants (Bishop et al. 2020; Zhou et al. 2021). Analysis of long term atmospheric Hg(0) and CO2 concentrations are very informative to understand the role of the vegetation in the global Hg cycle (Jiskra et al. 2018). Emission reduction measures adopted in Europe and North America since the 70s are corroborated by Hg dendrochemistry analysis showing a declining Hg concentration trend from the older to newer tree rings (Yanai et al., 2020). Indeed, tree ring Hg (dendrochronology) is a powerful archiving tool for atmospheric Hg(0). After Hg(0) oxidation inside the leaves, Hg(II) bind to organic compounds and then is transported fo the bole wood via the phloem (Beaufort et al., 1977; Lindberg et al., 1979). This is corroborated by the recent study of McLagan et al. (2022) showing the benefit of the stable Hg isotope analysis on dendrochrmistry.
Lines 76-77: I would suggest to add McLagan et al., (2022: https://doi.org/10.5194/bg-2022-124, recently accepted) to this reference on Hg dendrochemistry (using stable Hg isotopes). Some of the findings in this recently accepted study may be highly beneficial to this manuscript. We thank you for the reference. We added it as shown in the previous section.
Lines 102-103: I cannot agree with this statement that roots are the primary source of Hg in contaminated areas. (1) This is unpublished work and judging from the abstract it appears they state the atmosphere as the source not the roots; (2) This is at a former Hg mine – there is MASSIVE legacy emissions of Hg(0) to the atmosphere continuing to this day at these sites, which is readily available for stomatal assimilation; (3) as previously mentioned there is countless studies during the past 50 years that show root uptake in tree ubiquitously is an very minor, if not insignificant uptake pathway. This statements needs correcting. This statement is removed.
Lines 107-108: How does this compare to recommended soil guidelines? Please state this. This phrase is corrected as follows “Adding to that, soil samples collected from different areas in southern Lebanon showed values of Hg concentration ranging between 160-6480 ng/g showing a high contamination level as indicated by World reference Senesi et al. 1999, Kabata-Pendias 2001 ”.
Lines 144: I really don’t see the benefit of making acronyms of the sampling sites. Both a one-word towns and this just confuses readers that are not as closely linked to the study as the authors. I recommend simply writing the town names each time. We agreed and changed the acronyms to town names.
Figure 1: The climate graphs are really ancillary metadata. These are described in the method text and should be moved to the SI. Indeed, even the site map could be move to the supplementary information (SI). There are only two studies sites and again their location, climate and geography and surrounding Hg sources are described in the text. I believe this whole figure would be better served in a SI. On the behalf of all the authors, we believe it is better for the reader to have those sites indicated directly after the text to make it clearer concerning the locations. Concerning the climatic data, it is removed since it is available in the text and also indicated in figure 3.
Line 177: “8-15 m foot circumference”. There are obvious errors here. Also I highly recommend using diameter rather than circumference. It is much easier for the reader to comprehend. This sentence has been changed to: “For the Hg concentration analysis, four olive trees (3 to 5 m diameter and an average height of the trees 4-6m) were sampled in each of the two groves”
Line 196-197: While I do not think this is a major problem as I believe there will be minimal Hg(0) on surfaces or within foliage and stems, it needs to be acknowledge that this heated drying method would likely eliminated any and ALL Hg(0) present in the samples. This statement was amended to the following “Collected foliage and stems were rinsed with distilled water and then dried for 48 hours in an oven at a temperature of 50°C at maximum in order to remove any dust Hg from the surface of the samples following the method of Demers et al. 2013, Li et al. 2017, Pleijel et al. (2021). This procedure likely eliminate any Hg(0) present in the samples”.
Line 217: The detection limit should be listed as total mass of Hg, not concentration of Hg. The system does not analyse concentration (that is calculated by the mass of sample input), it is calibrated to determine the mass of Hg in any given sample. This is an important distinction. The calculation yielded the MDL of 0.04ng in units of mass (corresponding to a concentration of 0.7 ng g−1). This value was in the range of our measured Hg concentrations, which ranged from 0.4 ng for stem, 0.7 ng for soil and 0.8 ng for leaves and litter.
Line 261-263: What Hg concentration is being referred to here? Hg(0) Concentration in the air? This needs to be stated. This ambiguity is exactly why the results and discussion should be combined. The Hg concentration referred to here is the Hg content in the foliage and stems and it is the combination of Hg remaining in the foliage and stems after being rinsed and heated.
Lines 312: “the main Hg content” should be changed to “Highest Hg concentration”. This phrase is corrected as follows: “Our data corroborates previous studies (Bargagli 1995; Higueras et al. 2016; Naharro et al. 2018) showing that olive foliage has the highest Hg concentration of plant tissue.
Lines 318-322: Once again, I disagree the main source of Hg in the stems of the trees is from the soils. What is the evidence for this? I could reference 10+ papers that have shown Hg in woody materials of trees to be almost exclusively derived from foliage and downward transport in phloem. Higher concentrations in leaves over stems is NOT evidence that Hg in stems is derived from the roots. Not all Hg taken up by foliage is transferred to woody materials, which leads to an enrichment of Hg in foliage compared to Hg in woody materials. The phrase was amended to the following “Since normally the main source of Hg into the foliage is atmospheric and minimally through the soil, this explains the higher Hg concentration in the foliage, while in the stems the main uptake is from the foliage and transported through the phloem and soil, and it seems to be much lower levels of Hg concentration are found in the stems than that in the foliage (Pant et al., 2010; Tomiyasu et al., 2005).”
Lines 323-324: The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils. This phrase is added: “The downward transport of Hg in phloem eventually into roots and potential release into soils may also be contributing to Hg accumulation in soils.”
Lines 333-335: This concept of soil properties driving Hg concentrations and uptake in soils was not something introduced by O’Connor et al., 2019. This is not a new idea and again has been known of for decades. I agree, but I am giving here an updated reference.
Line 336: What does nitrogen content have to do with Hg sorption and uptake in soils? Nitrogen can also be a factor affecting the Hg content in soil depending on its characteristics. Nitrogen supply prevents oxidative stress in roots, but also can improve root development and increase the uptake of Hg from the soil (Carrasco-Gil et al. 2012).
Lines 339-341: Did the authors measure wet and dry deposition of Hg? For wet deposition to occur, there would need to be considerable Hg(II) in the atmosphere. I also see no reason as to why dry deposition would occur more in a higher temperature region. Higher temperatures favour partitioning of Hg(0) back into the gas phase, which would be suggestive of less dry deposition. The author discussed dry deposition and indicated the effect of higher temperature on higher dry deposition.
Lines 342-344: Foliage accumulates Hg(0) over time. Naturally older leaves that eventually die and senesce will be more enriched in Hg than younger leaves growing on the trees. I agree with your statement, that is why we indicated that older leaves have higher Hg concentration than younger leaves.
Lines 348-404 and Figure 3: This is far too speculative. These data are for Europe (the Authors use a site from Germany for Hg(0) in Figure 3). Lebanon is a long way from Europe and in a totally different climatic zone without typical northern temperate/boreal deciduous/conifer dominated forests. To make any sort of statement about atmospheric Hg zero concentrations this should have been measured or data taken from a long-term monitoring station in this climatic region/ecological biome. I see this whole (and very long discussion) on correlations between foliage and atmospheric Hg(0) to be too speculative to the point it is invalid. I also agree with the other reviewer that the emergence of foliage in olive trees in spring/early summer is the major driver here.
In the process of the reply to the review we rechecked the papers available on the atmospheric mercury closer to our region. We were not able to find any information in the region on atmospheric mercury data, but we passed across an interesting newly published paper by Martino et al. 2022 that is the only atmospheric gaseous elemental mercury (GEM) measurement near an olive site that is comparable to our study. Martino et al. 2022 publication shows the factors affecting the GEM values are mainly the factories, sea emissions, fires, and vegetation. Even though this paper is not in the same area as that of our sites, we take profit to compare with same years studied (2019-2020). We used their data from table 1 representing the monthly median GEM values and compared it to our data. Our values of the foliage in Bchaaleh and Kawkaba show higher values of Hg concentration in winter and lower in summer, were seasonality is clear the same as that of Martino et al. 2022. This seasonality can change due to many factors such as wind, fires, industrial surroundings and other factors. At the same time we decided to remove the northern Hemesphere data upon your feedback. We will change the discussion and narrow it to the Mediterranean region with the data mentioned in our comments.
Other data was done on Total Gaseous Mercury (TGM) in the French coastal Mediterranean site which showed higher values in winter/spring and lower values in summer in the Mediterranean region. This studies also showed seasonality with the atmospheric mercury due to dispersion of pollutants in the troposphere and high TGM values that are due to air masses from regional and local sources (Marusczak et al. 2015).
Mastromonaco et al. 2017 Showed also that the main source of mercury occurs from the Mediterranean Sea water and is affected by wind speed with higher values of GEM concentration registered in autumn in comparison to summer, with higher evasion rates found in summer in comparison to the autumn season. Another study that includes countries closer to Lebanon by Kotnik et al. 2014 found a seasonal variation in Hg in the Mediterranean region near Spain, Italy, Turkey and Israel (1998-1999) with the highest Hg average concentration during winter and the lowest during autumn.
Concerning the emergence of foliage in olive trees in spring/early summer being a major driver, this is shown through the seasonality registered during the different seasons due to the merging of three different years of the foliage in our study.
Lines 417-448: These paragraphs need grammatical and English language corrections. It is very hard to follow and from what I can derive it again seems highly speculative and to contradict the state of the science without data to support that. These paragraphs were checked by a scientific English editor. Some data was given to support our scientific point of view, but we tried to give our point of view in regards to the Hg cycling in stems, litter and soil system.
Citation: https://doi.org/10.5194/egusphere-2022-174-AC4
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AC4: 'Reply on AC3', Nagham Tabaja, 12 Oct 2022
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Nagham Tabaja
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Milad El Riachy
Ilham Bentaleb
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